A Science Case for an extended LOFAR

Transcription

1 A Science Case for an extended LOFAR edited by Corina Vogt - ASTRON, Dwingeloo, The Netherlands September 28, 2006 The purpose of this document is to summarise the scientific arguments from the European radio community for the extension of LOFAR as it is currently constructed in the Netherlands. LOFAR in its current extensions of about 100km baselines can achieve 12.6 arcsec resolution at an observation frequency of 60MHz whereas a LOFAR extending over 1000km can achieve resolution of 1 arcsec observing at the same frequency. Thus, the extension of LOFAR would not only increase the distribution of LOFAR considerably it would also enable unprecedented high resolution radio observations at low frequencies. This document is the result of contributions received from participants of a workshop which was held in May 2006 in Dwingeloo, The Netherlands. 1

2 Contents 1 Introduction LOFAR An extended LOFAR The Solar system - The Sun and the planets Solar observations with LOFAR Radio imaging of Jupiter s (and other planets) radiation belts Radio imaging of high latitude (auroral) magnetospheric emission from Jupiter Radio imaging of Solar system planetary lightning The very nearby Universe High-resolution low-frequency mapping of the Milky Way RM survey and high-resolution polarisation observations with LOFAR Radio emission from the intracluster medium Jets, their evolution and constituents Collimated outflows and galaxy evolution LOFAR and the lepton energy distribution in radio sources Low-luminosity extragalactic radio jets with extended LOFAR The high redshift Universe Extragalactic surveys with LOFAR long baselines Deep fields and high redshift star-forming galaxies Ultra-steep spectrum sources with an extended LOFAR LOFAR and associated H I in high-z radio sources Strong gravitational lensing Introduction Lens surveys Cluster lensing

3 1 Introduction 1.1 LOFAR The LOw Frequency ARray (LOFAR) - is a new generation radio telescope. Its ultimate mission is to survey the Universe at low frequencies with unprecedented resolution and sensitivity. It operates at those rarely explored radio frequencies from 30MHz to 240 MHz and with that LOFAR will open an almost ignored window in the frequency spectrum. Since mankind has not looked through this window with LOFAR s precision yet, LOFAR is almost bound to detect new exciting objects as other telescopes have done in the past when pushing open new observation windows. LOFAR belongs to a new generation of telescopes since its concept is to employ a lot of inexpensive dipole antennas arranged within stations without moving parts as one is used to from the big radio dishes such as the Westerbork Synthesis Imaging telescope. LOFAR observes different directions by steering the signal sensitive part the so called beam electronically across the sky and uses the rapidly growing computing power to process the digital signals and produce the final images of the low frequency sky. Observing at these low frequencies poses the challenge that the Earth s ionosphere heavily interferes with the radio observations. Separating these ionospheric effects from the real signal is one of the important issues to be solved. New developments in technology, such as improved antenna design, new concepts for the correction of the ionospheric jitter and the increasingly available processing power are key factors to the solution of this challenge. The new antenna designs make multi-beaming possible (observing in more than one direction at a time by using digital copies of a signal) and ultimately monitoring the sky all the time. The computer power is needed for processing the signals on a short enough time scale to correct for the ionospheric changes happening on short time scales. The spatial resolution of a radio telescope improves by increasing the separation between elements of the array, i.e. for LOFAR between stations, which is equivalent to increasing the baseline lengths. In its initial design, LOFAR was planned to facilitate station separations of up to 400km. In a first phase currently funded, LOFAR is built to cover at least 100km baselines spread over the Netherlands. The resolutions for its current extensions are given in Table 1.1. Table 1.1: Resolutions achievable with the core of LOFAR and the full array as it is currently assembled in the Netherlands. The core of LOFAR is planned to cover a 2x3km area. The full array will be spread over 100km. ν = 30 MHz ν = 75 MHz ν = 120 MHz ν = 240 MHz λ 10m λ 3.3m λ 2.5m λ 1.25m core array For the first phase accommodating baselines of up to 100km, the scientific drivers have been identified by the Astronomical community in the Netherlands 3

4 to detect the signal of the epoch of re-ionisation, to perform extragalactic surveys, to detect and to characterise new and well known transient phenomena, to perform pulsar surveys and to study their properties and finally, to detect the radio signals from cosmic ray interaction within the atmosphere. A detailed description of the science goals can be found elsewhere ( 1.2 An extended LOFAR LOFAR as it is realised across the Netherlands will reach what is called confusion limit rather easily especially at the lowest frequencies. This confusion limitation is produced by faint unresolved sources which produce a noise background. Any attempts to resolve these sources and look deeper will not be possible. Increasing the resolution will resolve more and more of these faint sources and allows us to glimpse more and more detail of the low frequency sky. Although the LOFAR telescope spread over the Netherlands will achieve unprecedented resolution at these low frequencies and will shed light on a lot of interesting problems, it would be fairly easy to extend LOFAR even further with a relatively small number of stations (of order 10 20) to achieve 10 times better resolutions. Discussions with various groups across Europe made it clear that there is widespread support for such an extension. The Max-Planck Institute in Bonn has started discussions with LOFAR already and is preparing to build a station at the site of the Effelsberg telescope. Other European institutes and universities are hoping to follow this example and discussions with various international partners in Germany, UK, France, Sweden, Italy and Poland have already been started. These developments have been received with enthusiasm by the Astronomical community in the Netherlands. An extended LOFAR reaching into Germany, UK, France, Sweden and Italy will be able to observe the radio sky at to reach sub-arcsecond resolution for at least the observing frequencies between MHz. An artists view of such an extended LOFAR is shown in Figure 1.1, although this view does not represent the actual potential European LOFAR sites. Table 1.2 summarises the resolutions achievable with an extended LOFAR. Table 1.2: Resolutions achievable with a LOFAR extended to station separations of 150, 500, 1000km. ν = 30 MHz ν = 75 MHz ν = 120 MHz ν = 240 MHz λ 10m λ 4m λ 2.5m λ 1.25m 150 km km km The spatial resolutions achievable will make new fundamental studies possible. It will deepen our understanding of objects in the neighbourhood of our planet, the nearby and the distant Universe: In the neighbourhood of our planet, the extended LOFAR will monitor Solar and Jovian outbursts, image the Sun, the radiation belts of the 4

5 Figure 1.1: An artists view on an extended LOFAR. Please note that this picture does not represent the actual long baseline planning. planets of the Solar system and detect lightening on other planets than the Earth, In the nearby Universe, the extended LOFAR will map the H II regions in the Milky Way, study polarisation properties of the Milky Way and detect weak magnetic fields in the intergalactic and extragalactic medium. In the nearby Universe, the extended LOFAR will shed light on the properties of the low-energy tail of the relativistic electrons contained in Jets, radio hotspots, AGN s and supermassive black holes. In the distant Universe, the extended LOFAR will allow us to survey large parts of the low frequency sky and detect in particular numerous star forming galaxies, to study the history and properties of radio sources throughout the Universe. It will also detect numerous lensed objects and allow us to study intrinsically faint star forming systems in the very distant Universe. In the very distant Universe, the extended LOFAR will detect very distant luminous radio sources and detect and study the origin of H I in very distant radio sources. These science drivers will be discussed in more detail below. Additional science will inevitably emerge by exploring new parameter space. 5

6 2 The Solar system - The Sun and the planets 2.1 Solar observations with LOFAR G. Mann, C. Vocks, Astrophysikalisches Institut Potsdam The Sun as our closest star is an intense source of radio waves. The already strong thermal radiation of the quiet Sun is superimposed by intense radio bursts that are associated with the solar activity. Sudden releases of an enormous amount of energy, called flares, lead to enhanced fluxes of energetic particles and electromagnetic radiation from the radio up to the gamma-ray range. Coronal mass ejections (CMEs) are launched from the solar corona into interplanetary space. CMEs drive shock waves ahead of themselves, that are also sources of energetic particles. These phenomena of solar activity can influence our Earth s environment, and, consequently, our technical civilisation. For instance, if a CME impinges on the Earth s magnetosphere, it causes disturbances of Earth s magnetic field. This impact of the solar activity on the Earth is usually referred to as Space Weather. The understanding and forecast of it is of great socioeconomic interest. The non-thermal solar radio radiation is plasma emission that is generated at the plasma frequency, f p = Ne 2 /(m e ɛ o )/(2π). It only depends on the electron number density at the source location, N, and natural constants. Thus, with a heliospheric density model it is possible to map coronal heights to radio frequencies. Figure 2.1 shows an example of a dynamic spectrum of the solar radio radiation. With a density model at hand, it is possible to derive the (phase-) velocity of the source from the frequency drift rate. Figure 2.1: Right: Dynamic radio spectrum observed on 18 July 1995 by the AIP s Observatory for Solar Radioastronomy at Tremsdorf. Left: Sketch of a coronal density model. But such observations provide no information on the spatial structure of the source. The Sun is only seen as a point source. LOFAR observations with long baselines provide an imaging capability with a theoretical resolution limit of a few arcseconds. Figure 2.2 demonstrates this improvement of image quality. Since radio emission in the LOFAR frequency range originates in the upper solar corona, LOFAR observations of nascent CMEs allow an assessment of their potential impact on the Earth s magnetosphere. 6

7 Figure 2.2: Improvement of the image resolution with LOFAR. The right picture was taken by the EIT instrument on-board the SOHO spacecraft. For LOFAR observations of the Sun, a number of modes are proposed. The routine task will be continuous monitoring of the Sun with radio images in selected frequencies every 3 minutes. As soon as a radio burst is detected ( burst bell ), follow-up observations can be triggered automatically. Furthermore, joint observations campaigns can be organised with other ground- and space-based instruments, like STEREO, SDO, Solar-B, or the Solar Orbiter. Such observations cover the whole electromagnetic spectrum, from radio frequencies to EUV and X-rays. 2.2 Radio imaging of Jupiter s (and other planets) radiation belts Philippe Zarka, Observatoire de Paris - CNRS, LESIA, France Jupiter s radiation belts power synchrotron emission over the meter to decimetre range. This radiation has already been imaged in the decimetre range by many ground-based instruments (Figure 2.3), and historically has given the first information about Jupiter s quasi-dipolar magnetic field and energetic (MeV) electron populations in the inner magnetosphere (de Pater, 2004, and references therein). High spatial resolution imaging below 300 MHz of the sources of such emission (and their time variability with planetary rotation and solar wind fluctuations) remains to be done. LOFAR will address this question over a large relative bandwidth, allowing us to study the origin, transport, scattering (by plasma waves, Coulomb scattering), and loss (through synchrotron emission or by interaction with dust) of high energy electrons in Jupiter s inner radiation belts (de Pater, 2004, and references therein). No synchrotron emission has been detected from Saturn, due to the absorption of energetic particles by the rings (Van Allen and Grosskreutz, 1989). A deep search for a weak radiation belt might be attempted. Bursts of energetic electrons were observed by the Mariner 10 mission in the magnetosphere of Mercury (Baker, 1986). These might cause bursts of synchrotron emission (there was no radio instrument on Mariner 10 to confirm this). High angular resolution imaging will be required at selected frequencies (i.e. in 4 MHz bands at selected centre frequencies) between 50 and 240 MHz. The resolution offered by 100 km baselines is sufficient in LOFAR high-band, but extended baselines will be necessary 100 MHz. Image or (u,v) cubes with a few 7

8 Figure 2.3: Left: Radio image of Jupiter s decimetric emission at a wavelength of 20 cm, and a central meridian longitude of 312, taken at the VLA in June The resolution is 0.3 R J. The main radiation peaks are indicated by the letters L and R, and the high latitude emission peaks by Ln, Ls, Rn and Rs. Model magnetic field lines with apex at 1.5 and 2.5 R J are superposed. Field lines are shown every 15. Right: Radio contour map of Jupiter s linearly polarised flux density at 15 GHz, averaged over the planet s rotation (the changing viewing aspect due to Jupiter s rotation was properly accounted for). Superposed are the electric vectors rotated by 90, tracing out the planet s magnetic field as projected on the sky. The length of the vectors are proportional to the polarised flux density. Contours start at 4σ (1σ = 0.5 mjy/beam), and contour intervals are in steps of 3σ. The beam size (HPFW) is indicated in the lower left corner (from de Pater, 2004). 10 s of seconds time resolution are adapted. Intensity of emission is expected to be in the range 1 5 Jy. Full polarisation is needed (linear polarisation perpendicular to local B-field expected). For Saturn and Mercury s radiation belts and possible synchrotron emission, first observations should aim at detection only, and thus not require very high angular resolution. Lower frequencies than Jupiter s case should be observed (probably MHz). In case of detection, then high resolution follow-up studies would be relevant. References [1] Baker D. N., Jovian electron populations in the magnetosphere of Mercury, Geophys. Res. Lett., 13, 789, 1986 [2] de Pater I., LOFAR and Jupiter s radio (synchrotron) emissions, Planet. Space Sci., 52, 1449, 2004 [3] van Allen J. A., Grosskreutz C. L, Relativistic electrons in Saturn s inner magnetosphere and an estimate of their synchrotron emission, J. Geophys. Res., 94, 8731,

9 2.3 Radio imaging of high latitude (auroral) magnetospheric emission from Jupiter Philippe Zarka, Observatoire de Paris - CNRS, LESIA, France An extended LOFAR offers the possibility to image with time resolution of a few milliseconds the sources of Jupiter s high latitude decametric radio emissions ( 40 MHz) which are due to the magnetosphere interaction with the solar wind and with the Galilean satellites, primarily Io (Figure 2.4; Zarka 2002, Zarka 2004, Zarka 2005). Many spectral studies have been carried out from ground and space-based instruments (Figure 2.5), but imaging at low frequencies remains to be done. As explained in Zarka (2004), it will bring a wealth of fundamental information on Jupiter s high- latitude/mild energy (kev) electrons. If a resolution of 1 2 can be achieved at MHz (requiring LOFAR baselines of up to 1000 km), then the observations could bring new information on the physics of the radio generation process (beaming angle of the radiation relative to the local magnetic field, as a function of frequency), on Jupiter s surface magnetic field (direct mapping of instantaneous cyclotron sources of highest frequency), on the Io Jupiter electrodynamic interaction, as well as that with the other Galilean satellites (lead angles between the satellite flux tube and the radio emitting field line), and on the Io plasma torus (via measurements of Faraday rotation and diffraction fringes due to radio wave propagation through the torus). Fast imaging (at millisecond timescale) is made possible by the very high intensity of Jovian decametre bursts (up to several million Jansky as seen from the Earth), which could allow us to follow electron bunches and thus measure potential distributions along the Io flux tube (Hess et al. 2006). Combined with observations at other wavelengths (radio, ultraviolet, infrared, X-ray) and addressing time variability, LOFAR s fast imaging capabilities should be of major interest for the study of Jupiter s magnetospheric structure and dynamics. Highest possible angular resolution imaging is required at lowest possible frequencies below 40 MHz (i.e. down to 30 MHz or even down to 10 MHz), with spectral resolution of khz. Some objectives are doable with moderate ( km) baselines (e.g. Faraday probing of Io s plasma torus), but most of them will require higher resolution (say 4 ) and thus extended baselines 500 km. Image or (u,v) cubes with 1 sec time resolution will be adapted to most objectives, except for short bursts due to electron bunches moving along Io s flux tube, for which series of images (or (u,v) maps) at millisecond time resolution are requested. Emission may be very intense, with bursts up to Jy and dynamic range >20 30 db. Full polarisation (4 Stokes parameters) is essential for identifying emission mode, for Faraday studies, etc. Ability to do interferometry at 800 km distance on a regular basis has been recently tested successfully (after early studies by Dulk, 1970 and others) through VLBI-like observations of Jupiter between LOFAR-ITS and Nançay decametre array (Zarka, Nigl et al., in preparation). Quantitative assessment of this ability versus time and frequency is in progress. Magnetised exoplanets are also expected to produce intense decametric radio emissions, but LOFAR will first address their detection, which requires high sensitivity (Farrell et al., 2004). Radio astrometric high resolution measurements of exoplanets do not appear to be a strong scientific driver for an extended LOFAR. 9

10 Figure 2.4: Top left: Sketch of radio source locations in the Jovian magnetosphere. bkom, HOM and DAM are auroral emissions generated near the local electron gyrofrequency f ce and beamed in widely opened hollow cones aligned on magnetic field lines with various latitudes. These high-latitude emissions actually exist in both hemispheres. nkom is emitted in Io s plasma torus. Left inset sketches the Io-Jupiter interaction through Alfvén waves, which results in strong radio emission (Io-DAM) detected when Io s phase is A or B. Right inset sketches the correspondence of radio sources with UV ones (main oval and Io s spots and trail). Only DAM emission is detectable from the ground. (Bottom left) Dynamic spectrum of a typical Io-DAM emission recorded on 1/1/1991 in Nançay, in right-hand circular polarisation, with time resolution 1 sec/spectrum. Horizontal lines are man-made interference, vertical lines are calibrations. Bottom right: High resolution dynamic spectrum of Jovian short bursts recorded at Nançay using an Acousto-Optocal-Spectrograph, with resolution of 3 msec/spectrum. References [1] Dulk G. A., Characteristics of Jupiter s decametric radio source measured with arc-second resolution, Astrophys. J., 671, 1970 [2] Farrell W.M., et al., The Radio Search for Extrasolar Planets with LOFAR, Planet. Space Sci., 52, 1469, 2004 [3] Hess S., Zarka P., Mottez F., Io-Jupiter interaction, millisecond bursts and field aligned potentials, Planet. Space Sci., in press, 2006 [4] Ladreiter H. P., Zarka P., Lecacheux A., Direction-finding study of Jovian hectometric and broadband kilometric radio emissions: Evidence for their auroral origin, Planet. Space Sci., 42, 919, 1994 [5] Zarka P., Planetary Science with the Low Frequency Array (LOFAR), Proceedings of the URSI General Assembly, 2078, 2002 [6] Zarka P., Planetary Low Frequency Radio Astronomy with large groundbased instruments, Proceedings of the URSI General Assembly, Invited paper J05-I-COM ,

11 Figure 2.5: Goniometric results from Ulysses near Jupiter on 1994/02/08, from 12 R J from the planet. (a) The direction of the dominant hectometre RH circular source at 387 khz measured during 12-sec intervals over a 35 minute-period is plotted as open dots in the frame of Ulysses radio antennas. Jupiter is represented with its rotation axis (lightface segment) and its magnetic axis (boldface line) pointing to the North. Dotted lines are dipole magnetic field lines with apex at 6 and 12 R J. Auroral ovals crossing these field lines at a local electron cyclotron frequency f ce=387 khz are solid/dashed lines (visible/hidden portion from Ulysses). (b) Average source directions over the same 35-min interval at 387, 540, and 740 khz. The dotted dipolar field line with apex at 6 R J crosses the 3 radio sources at the same point as the corresponding northern auroral ovals along which f ce= 387, 540, and 740 khz with increasing distance from the planet (solid/dashed). This measurement by Ladreiter et al. (1994) is the first direct proof that hectometre emission is produced at/near the local f ce on the X mode (RH polarisation from northern magnetic hemisphere). The geometry allows to deduce the 3D source location and radio beaming angle. [7] Zarka P., Fast radio imaging of Jupiter s magnetosphere at low frequencies with LOFAR, Planet. Space Sci., 52, 1455, Radio imaging of Solar system planetary lightning Philippe Zarka, Observatoire de Paris - CNRS, LESIA, France Lightning is a transient, tortuous high-current electrostatic discharge resulting from macroscopic electric charge separation (by convection and gravitation combined), following small-scale particle electrification (via collisions and charge transfer). A large-scale electric field builds up, which may eventually lead to breakdown and ionise the intervening medium, causing a lightning stroke. A lightning discharge consists of many consecutive strokes and lasts typically msec. It may have an important role in the atmospheric chemistry (production of non-equilibrium trace organic constituents, potentially important for biological processes). Electromagnetic signatures include optical, VLF and LF radio emissions. 11

12 Figure 2.6: Left: Dynamic spectrum of Saturn s lightning obtained during Voyager 1 fly-by 1 hour after closest approach, from a range of 5 planetary radii. The radio instrument on-board Voyager was a swept-frequency analyser, thus detecting impulsive broadband phenomena as short streaks parallel to the frequency axis, randomly distributed over their whole spectrum (inset). Right: Similar dynamic spectrum recorded 1 hour after closest approach from Uranus. White areas are data gaps. Dark horizontal (and a few vertical) lines are spacecraft interference (in both panels). Lightning on Saturn (Warwick et al., 1981) and Uranus (Zarka and Pedersen, 1986) have been discovered by the Voyager spacecraft (Figure 2.6). Their study has been proved to be possible using large ground-based instruments (Zarka et al. 2004, 2005, 2006), which may also be used to assess the existence of lightning in the thick atmosphere of Venus, in Martian dust clouds (dust devils), and on Neptune. Tentative detections of Saturn s lightning have been carried out with decametres arrays in Ukraine (Zarka et al. 1997) and France (Zarka et al. 2006) with encouraging but as-yet inconclusive results. LOFAR will have powerful enough capabilities to settle the matter (Figure 2.7 ; Zarka et al. 2004). Monitoring of planetary lightning (possibly in correlation with optical surveys) will allow the study of electrification processes (breakdown electric field build-up which depends on atmospheric pressure, composition, electron density), atmospheric dynamics and cloud structure), of geographical and seasonal variations, and the comparison with the same processes on Earth. High angular resolution imaging in 4 MHz bands at MHz and if possible at lower frequencies (down to 10 MHz) is required, because planetary diameters as seen from Earth are <1 arcmin. For the search of Martian discharges, frequencies of interest are in the whole LOFAR band (up to 240 MHz). Arcsec resolution (extended full array) is required to resolve the planetary disks, but lower resolution is enough at a first stage to confirm the planetary origin of the lightning bursts. Time resolution should be of the order of msec (typical flash duration). Emission is intrinsically broadband so spectral resolution 1 4 MHz is enough for the final products (besides RFI mitigation needs). At least 12

13 Figure 2.7: Summary of planetary lightning spectra (observed or predicted) compared with expected LOFAR sensitivity. The light grey-shaded region is the sky background noise spectrum that will be detected by LOFAR, depending on the configuration used (virtual core only or full array). The darker grey-shaded regions characterise the noise background fluctuations that will define the sensitivity (1σ) of the observations with a bandwidth of 4 MHz and an integration time of 1 or 100 msec. Boldface planetary lightning spectra (for Saturn, Uranus, and Neptune) correspond to extrapolations of Voyager observations. Lightface lines represent upper limits or models of unobserved planetary lightning radio spectra. The range accessible to ground-based observation is at the right of the dashed line (ionospheric cutoff). two 4 MHz bands should be observed simultaneously to confirm the broadbandness of the emissions. Expected fluxes are in the range 10 µjy to 100 Jy at Earth, depending on the target. Emissions are expected to be unpolarised (this can be checked through polarisation measurements). Lightning from exoplanets should be too weak to be detectable from the Earth. References [1] Warwick J. W., et al., Planetary Radio Astronomy observations from Voyager 1 near Saturn, Science, 212, 239, 1981 [2] Zarka P., Pedersen B. M., Radio detection of Uranian lightning by Voyager 2, Nature, 323, 605, 1986 [3] Zarka P., et al., Ground-Based High Sensitivity Radio Astronomy at Decameter Wavelengths, in Planetary Radio Emissions IV, H.O. Rucker et al. Eds., Vienna: Austrian Acad. Sci., pp. 101,

14 [4] Zarka P., Farrell W. M., Kaiser M. L., Blanc E., Kurth W. S., Study of solar system planetary lightning with LOFAR, Planet. Space Sci., 52, 1435, 2004 [5] Zarka P., Planetary Low Frequency Radio Astronomy with large groundbased instruments, Proceedings of the URSI General Assembly, Invited paper J05-I-COM , 2005 [6] Zarka P., et al., Physical Properties and Detection of Saturn s Lightning Radio Bursts, in Planetary Radio Emissions VI, H.O. Rucker et al. Eds., Vienna: Austrian Acad. Sci., pp. 111,

15 3 The very nearby Universe 3.1 High-resolution low-frequency mapping of the Milky Way Wolfgang Reich, Max-Planck-Institut für Radioastronomie, Bonn,Germany An extended LOFAR with baselines up to a few hundred kilometre will largely improve the angular resolution of the Dutch LOFAR array and opens new possibilities for astronomical research. Low-frequency observations of Galactic emission has already been described in the German LOFAR White Paper [1], which complement current and future high-frequency results. Galactic research largely benefits from high angular resolution mapping by resolving confusion problems, which are believed to be the limiting factor for a number of continuum and polarisation observations. Galactic sources: High-resolution multi-frequency mapping of Galactic supernova remnants (SNR) are of interest for detailed spectral and intrinsic absorption studies. Source scattering near SNR shocks, in particular, requires high angular resolution. Recent 74 MHz observations of small Galactic plane sections with the VLA revealed an unexpected large number of new shell-type SNRs [2], although for a number of objects the angular resolution of 42 is clearly insufficient to resolve confusion with surrounding complex emission structures for a clear identification. Plerionic or Crab-like SNRs have a high internal magnetic field and their break-frequency is supposed to move quickly towards low frequencies, which might explain their small number among the identified SNRs. For the identification of diffuse flat-spectrum objects at low frequencies a separation of extragalactic steep-spectrum sources is essential to make these observations successful. H II regions become optically thick at low frequencies. For a study of their internal density and electron temperature distribution high angular resolution observations are needed. Galactic tomography: Low-frequency absorption towards H II regions at known distances can be used for a tomography of Galactic emission components. This powerful method was so far limited by the low angular resolution of low-frequency observations, that data for just a few extended H II regions are available (see for instance [3]). With a sufficiently large number of well resolved H II regions the 3D-distribution of cosmic rays, magnetic fields and the diffuse thermal gas in the Milky Way can be derived. A local synchrotron emission excess by about a factor of three was derived by Fleishman & Tokarev [4] based on low angular resolution absorption data towards large H II regions. This excess, however, needs to be investigated in all directions, as it clearly constrains models of the emissivity distribution of the Milky Way. In particular the halo or thick-disk component, which is currently believed to have a scale height of several kpc, might be rediscussed. LOFAR s sensitivity and spectral coverage will allow to study the low-frequency absorption of many hundreds of compact H II regions across the entire Galaxy and thus will provide the required key information on its emissivity distribution. High-angular resolution is expected to be increasingly important to resolve the weak absorption signals from far distant H II regions from the complex line of sight Galactic emission along the Galactic plane. Galactic polarimetry: High angular resolution is of particular impor- 15

16 tance for low-frequency polarimetry of the diffuse Galactic emission. It is well established that the vast majority of the rather numerous complex polarisation structures seen on actual polarisation surveys [5] have no correspondence in total intensity and are thus the result from Faraday rotation within the magneto-ionic interstellar medium. As Faraday rotation scales by λ 2 depolarisation along the line of sight and within the beam area increases largely towards low frequencies. Depolarisation limits the maximum distance from which polarised emission can be observed. The observed rotation measures of diffuse emission decreases towards low frequencies when compared with that measured for extragalactic sources, which trace the full line of sight through the Galaxy. High angular resolution increases the so called polarisation horizon at low frequencies. Polarimetry with rather narrow frequency channels is another need, which is provided by the LOFAR receiving and processing system. The extended LOFAR will allow studies of spatially resolved very small fluctuations of the strength and the direction of the interstellar magnetic field within a few hundred parsec. Also fluctuations of the thermal electron density can be measured via rotation measures with unprecedented sensitivity surpassing that of H α observations. The filling factor of thermal electron densities and the strength and the regularity of the magnetic field are important input parameters to model the magnetoionic interstellar medium. LOFAR s large field of view, wide frequency range split into rather narrow frequency channels and an angular resolution of a few arcseconds are ideal instrumental conditions for Galactic polarisation work with unprecedented quality. References [1] German LOFAR White Paper, eds. M. Brüggen et al., 2005, MPIfR Bonn [2] Brogan, C., Gelfand, J.D., Gaensler, B.M. et al: 2006, ApJ 639, L25 [3] Roger, R., Costain, C.H., Landecker, T.L. et al: 1999, A&AS 137, 7 [4] Fleishman, G.D., Tokarev, Yu.V.: 1995, A&A 293, 565 [5] Reich, W., 2006, in Cosmic Polarization, ed. R. Fabbri, Research Signpost, in press (astro-ph/ ) 3.2 RM survey and high-resolution polarisation observations with LOFAR Rainer Beck, Max-Planck Institut für Radioastronomie, Bonn, Germany The origin of magnetic fields in stars, galaxies and clusters is an open problem in astrophysics and fundamental physics. The SKA Key Science Project The origin and evolution of cosmic magnetism will perform an all-sky Faraday rotation measure (RM) survey at 1.4 GHz to model the three-dimensional structure and strength of the magnetic fields in the intergalactic medium and the interstellar medium of intervening galaxies and of the Milky Way [4]. It is expected to obtain RM values from polarised background sources with an accuracy of about 5 rad m 2. An RM survey in the LOFAR highband ( MHz) will be an ideal extension of the SKA survey to smaller rotation measures and can develop the analysis tools needed for an all-sky RM survey. With its wide-band polarisation 16

17 facility, LOFAR can detect rotation measures down to about 0.1 rad m 2 (giving 36 o total rotation at 120 MHz) and hence become the telescope to measure the weakest cosmic magnetic fields so far. RM values of 1 rad m 2 are expected from galaxy halos and cluster halos (field strength B 10 6 G, thermal electron density n e 10 3 cm 3, pathlength L 1 kpc). Intergalactic magnetic fields could be detected for the first time: Fields of B G are expected along filaments of 10 Mpc length with n e 10 5 cm 3 [7] which yield RM = rad m 2. Much higher rotation measures occur in intervening galaxies and in the Milky Way. In the 1.4 GHz DRAO/ATCA polarisation survey of the Galactic plane (1 resolution), RMs of background sources of several 100 rad m 2 were observed [2, 6], while in the MHz WSRT polarisation survey (5 resolution) of two fields near the plane the source density and RMs are lower ( RM 50 rad m 2 ) [5], probably due to depolarisation effects within the large beam. With LOFAR s large number of channels, spectro-polarimetry (RM synthesis) can measure almost the whole range of RM values and separate RM components from distinct foreground and background regions [1]. To avoid bandwidth depolarisation, the spectral resolution has to be better than ν/ν = 0.5/(λ 2 RM) (= 0.1/RM at 130 MHz) which will be achievable with LOFAR even for large RMs. Faraday depolarisation is a more serious problem at low frequencies. RM gradients in the Galactic foreground can be several 100 rad m 2 across the average source separation of one degree [2]. To avoid depolarisation, the RM gradients have to be resolved to better than 0.1 rad m 2 per beam and thus need about 1 resolution. Within the background sources (mostly distant compact radio galaxies), Faraday depolarisation can occur by RM gradients along the jets and beam depolarisation due to different polarisation angles of the unresolved jets. To resolve a jet of 1 kpc length at 100 Mpc distance, a resolution of 1 is needed. With this resolution and LOFAR s excellent sensitivity, it is expected to obtain a high number density of polarised background sources for the RM survey. Low-frequency radio emission traces low-energy cosmic ray electrons which suffer less from energy losses and can propagate much further away from their sources into regions with weak magnetic fields. The lifetime of electrons emitting in a magnetic field of about 1 µg is yr at 50 MHz and yr at 150 MHz, limited by inverse Compton loss with CMB photons. LOFAR will allow to map the structure of weak magnetic fields in the halos of galaxies and galaxy clusters and in the intergalactic medium, ideally supplementing the RM survey proposed above. Polarised emission outside galaxies is an ideal tracer of galaxy interactions (see Figure 3.1) and of ram pressure in galaxy clusters [8]. Again, avoiding depolarisation by gradients of Faraday rotation within the source and in the Galactic foreground needs high angular resolution. As it is aimed for galaxies and galaxy clusters with diameters of a few arcminutes, internal RM gradients are most severe. Resolving the typical scale of RM turbulence of 100 pc at 10 Mpc distance calls for 1 resolution at 150 MHz, similar to the requirement for the RM survey. Therefore maximum LOFAR baselines of 500 km are needed. 17

18 Figure 3.1: Polarized emission (contours) and B-vectors of the spiral galaxy NGC 4569 in the Virgo cluster, observed at 4.8 GHz with the Effelsberg 100-m telescope (from [3]). References [1] Brentjens, M. A., de Bruyn, A. G.: 2005, A&A 441, 1217 [2] Brown, J. C., Taylor, A. R., Jackel, B. J.: 2003, ApJS 145, 213 [3] Chyży, K. T., Soida, M., Bomans, D. J., et al.: 2006, A&A 447, 465 [4] Gaensler, B. M., Beck, R., Feretti, L.: 2004, in Science with the Square Kilometer Array, ed. C. Carilli & S. Rawlings (Amsterdam: Elsevier), New Astronomy Reviews 48, 1003 [5] Haverkorn, M., Katgert, P., de Bruyn, A.G.: 2003, A&A 403, 1031 and A&A 404, 233 [6] Haverkorn, M., Gaensler, B. M., Brown, J. C., et al.: 2006, AN 327, 483 [7] Kronberg, P. P.: 2006, AN 327, 517 [8] Vollmer, B., Beck, R., Kenney, J. D. P., van Gorkum, J. H.: 2004, AJ 127,

19 3.3 Radio emission from the intracluster medium Luigina Feretti, Istituto di Radioastronomia INAF, Via P. Gobetti 101, Bologna, Italy Diffuse radio sources An important aspect of the radio emission from galaxy clusters is represented by diffuse radio sources associated with the intra-cluster medium (ICM): radio halos, relics and mini-halos (e.g. Feretti 2005). These are extended sources, whose synchrotron nature indicates the presence of cluster-wide magnetic fields of the order of µg, and of a population of relativistic electrons with Lorentz factor γ >> Radio halos and relics are related to cluster mergers, whereas mini-halos are detected at the centre of cooling core clusters. The low surface brightness (< µjy arcsec 2 at 1.4 GHz) of these diffuse sources and their steep spectrum makes it difficult to image them accurately with the current resources, and implies the need of new generation radio telescopes at low frequencies. Owing to their large size, from hundreds kpc to > Mpc, these sources are easily detected at intermediate angular resolution. However, high resolution data are needed for the accurate determination of their properties. Imaging: High resolution imaging is important to derive information on the detailed structure of diffuse radio sources. Furthermore, at lower resolution, where beam averaging enhances the detectability of extended radio emission, it is sometimes difficult to distinguish true diffuse emission from a blend of weak, discrete radio sources. Ideally, one would need high sensitivity on all angular scales. The recognition and subtraction of unrelated sources, which is only possible when high resolution data are available, is crucial to derive reliable radio spectra of these sources. Low frequency (< 300 MHz) spectra are important to determine the index of electron energy distribution and to derive constraints on any re-acceleration process. In addition, spectral index maps represent a powerful tool to study the properties of the relativistic electrons and of the magnetic field in which they emit, and to investigate the connection between the electron energy and the ICM properties. By combining high resolution spectral information and X ray images it is possible to study the thermal relativistic plasma connection both on small scales (e.g. spectral index variations vs. clumps in the ICM distribution) and on large scales (e.g. radial spectral index trends). Polarisation: Relics are highly polarised (see e.g. Govoni & Feretti 2004); moreover polarised filaments have been detected in the radio halo of A2255 (Govoni et al. 2005). Polarisation studies are important to derive information on the orientation of cluster magnetic fields, their characteristic size-scale and power spectra. In the case of relics, which have been suggested to be tracers of shock waves in merger events, RM studies are promising also to analyse the cluster merger geometry (Clarke & Enßlin 2006). High resolution data are necessary to reduce beam depolarisation. Sub-arcsec resolution is needed to detect polarised structures on the kpc scale in clusters at z > 0.1. Moreover, foreground screens due e.g. to the host galaxy, local turbulence, clouds, etc., can be resolved, and the Faraday effect internal to the radio source can be disentangled from that of external origin. A powerful tool to these aims is the RM synthesis technique (Brentjens & de Bruyn 2005); in 19

20 addition, numerical simulations are very important for the interpretation of RM data. Thomson scattering The radiation emitted by an active nucleus can be scattered by the hot electrons if highly ionised material is present, as in the cluster cooling cores. A diagnostic of scattered radiation is polarisation. The scattered radiation reflects information both on the central source and the surrounding medium. Time variability of the central nucleus, beaming, or polarised emission all produce characteristic profiles for the scattered emission which can be used to study these processes. A very interesting case would be the detection of Thomson echo in a galaxy cluster containing no compact radio source; this might imply that a bright radio source existed in such cluster in the past. This is important to study the radio source duty cycle and its relation to the cluster cooling flow phenomenon. Since the scattered light is merely the emission of the central source redirected toward the observer, the diffuse scattered component will have the same spectral index as the central source. Thus, given data of high resolution and sufficient dynamic range, it should be possible to distinguish between scattered radio photons and those from other sources. Summary High resolution studies at low frequencies will add important information to the knowledge of extended diffuse radio sources in clusters, despite of their typical large size. In particular, the analysis of substructures, the accurate subtraction of discrete unrelated sources, the determination of spectra and spectral index maps will benefit from high resolution in total intensity studies. For polarisation studies, high resolution data are crucial to reduce beam depolarisation. This allows the investigation of magnetic field structures, the possibility of disentangling the internal from external Faraday effect, the discovery and study of Thomson scattered sources. References [1] Brentjens, M.A., de Bruyn, A.G., 2005, A&A 441, 1217 [2] Clarke, T.E., Enßlin, T.A., 2006 AJ 131, 2900 [3] Feretti, L., 2005 AdSpR, 36, 729 [4] Govoni, F., Feretti, L., 2004 IJMPD 13, 1549 [5] Govoni, F., Murgia, M., Feretti, L., Giovannini, G., Dallacasa, D., Taylor, G.B., 2005, A&A 430, L5 20

21 4 Jets, their evolution and constituents 4.1 Collimated outflows and galaxy evolution Anrei Lobanov, Max-Planck-Institut für Radioastronomie, Bonn, Germany Collimated outflows (jets) are found in a wide range of cosmic objects including our own Sun, brown dwarfs, planetary nebulae, proto-stars, young stellar objects (YSO), X-ray binaries, and galactic nuclei. In all of these objects, jets transport excess angular momentum and energy from the vicinity of compact, rotating objects into the interstellar and intergalactic medium. Kinetic feedback from outflows in AGN is recognised as one of the key factors for the formation of large scale structures in the Universe and evolution of galaxies and their central black holes. Non-thermal emission from stellar and proto-stellar outflows can hardly be detected at present and its nature and physical properties are poorly known. Collimated outflows from active galactic nuclei are expected to have two major components: a highly relativistic electron-positron spine and a substantial sub-relativistic outflow possibly containing a colder, electron-proton plasma [5]. The relativistic spine is likely to be substantially stratified [1, 6], with complex transverse structures in the particle density, magnetic field and velocity (both in the poloidal and toroidal components). The sub-relativistic component of the flow would carry most of the kinetic power vested into the outflow. Both the stratification of the relativistic outflows and the properties of the sub-relativistic flows are very difficult to study at centimetre wavelengths, since a large fraction of the moving plasma is expected to be prominent only at low frequencies The steep spectrum of the non-thermal emission from outflows makes observing in the MHz range equivalent to increasing sensitivity of centimetre wave observations by factors of Observations with LOFAR will therefore provide a unique opportunity to probe non-thermal, synchrotron emission produced by the low energy tail of the plasma in the outer layers of collimated outflows and in the regions with highly evolved plasma such as extended lobes, relics and cavities produced by the jet activity in the interstellar and intergalactic medium. There are several major areas of astrophysical research in which the lowfrequency information provided by observations with LOFAR is expected to produce a fundamental impact: 1. Formation and evolution of collimated outflows from galactic and extragalactic objects. LOFAR would bring new dimensions to studies of the structure and dynamics of the outflows and allow detailed investigations of their interaction with the external medium to be made. Low frequency observations will be essential for detecting and studying the outer layers of extragalactic jets where the effects of stratification and interaction with the ambient medium are most prominent (Figure 4.1). 2. Evolution of astrophysical plasmas. LOFAR would provide an outstanding foundation for detailed quantitative studies of evolution and re-acceleration of non-thermal plasma in cosmic objects. Measurements of polarisation will give reliable measures of the magnetic field distribution in the outflows, enabling investigations of the role it plays on extended scales, in galactic and extragalactic objects. In the extended regions of the flows, the synchrotron turnover is 21

22 Structures that will be explored by LOFAR Superluminal features Observable flow at cm wavelengths Figure 4.1: Simulated structure of a relativistic jet interacting with the external medium [3]. Present day observations at centimetre wavelengths can detect only a narrow fraction of the entire flow, with bright features moving at superluminal speeds. Some of the extended structures will be detectable at centimetre wavelengths with SKA, but the full extent and depth of detail of interaction between the flow and the external medium will only be observable at very low frequencies probed by LOFAR. expected to be found at frequencies far below 1 GHz, and multi-frequency LO- FAR observations will be used for direct imaging of the turnover frequency and magnetic field distribution [4]. 3. AGN feedback and its impact on the Universe at large. LOFAR studies of extragalactic flows will provide essential clues for understanding the power and efficiency of the kinetic feedback from AGN and its effect on activity cycles in galaxies and cosmological growth of supermassive black holes. LOFAR will enable making accurate estimates of the kinetic power of the outflows and its contribution to the heating of the IGM. Complemented with X-ray observations, this would provide a framework for estimating and comparing the relative efficiencies of the radiative and kinetic feedback mechanisms [2]. This assessment will be crucial for understanding the physical processes in the IGM and intracluster environment in the Universe. 4. Evolution of nuclear activity in galaxies. LOFAR observations will provide unsurpassed capabilities for detecting and studying the relics of galactic activity. LOFAR would be able to detect such relics for at least 10 7 years after the fuelling stops, and this would make it possible to assess the activity cycles in a large number of objects, searching for signs of re-started activity in radio-loud objects and investigating paleo activity in radio-quiet objects. This information will be essential for constructing much more detailed models of evolution and nuclear activity of galaxies. 5. Cosmological evolution of supermassive black holes The physical relationship between accretion on supermassive black holes (SMBH) and their energetic feedback to the surrounding medium (jets) can also be used to better uncover the cosmic evolution of SMBH and their host galaxies. LOFAR observations provide measures of the total kinetic power output of an AGN over its lifetime. A comparison between the inferred evolution of black hole masses with that of the relics of jet activity would provide fundamental clues about the mode in which black holes actually grew throughout their history, thus on the very same physics of the accretion/jet coupling. 22

23 A combination of these five areas makes a strong case for considering studies of collimated outflows one of the key scientific areas for LOFAR. References [1] Aloy, M.A., Gómez, J.L., Ibáñez, J.M., Martí, J.M., Müller, E. 2000, ApJ, 528, L25 [2] Brüggen, M., Hoeft, M., Ruszkowski, M. 2005, ApJ, 628, 153 [3] Duncan, G.C., Hughes, P.A. 1994, ApJ, 436, L119 [4] Lobanov, A.P. 1998, A&AS, 132, 261 [5] Sol, H., Pelletier, G., Asseo, E. 1989, MNRAS, 237, 411 [6] Swain, M.R., Bridle, A.H., Baum, S.A. 1998, ApJ, 507, L LOFAR and the lepton energy distribution in radio sources Gianfranco Brunetti, INAF - Istituto di Radioastronomia, Via P. Gobetti 101, Bologna, Italy Particle acceleration and energetics of extragalactic radio sources The synchrotron emission from extragalactic radio sources is powered by relativistic electrons (and positrons) radiating in magnetic fields with strength in the range µg. The origin of these particles is an issue of great interest in modern astrophysics. While stochastic re-acceleration mechanisms might play an important role in accelerating the emitting electrons in large scale (Mpc) radio sources (e.g. reviews by Sarazin 2002; Brunetti 2003,2004a; Lazarian 2006), the bulk of particles radiating in the classical radio galaxies and quasars are accelerated by shocks at the head of the relativistic jets or along the jets themselves (e.g., review by Meisenheimer 2003). The spectrum of the emitting electrons in radio sources is steep (i.e. δ 2, with N(E) E δ, α = (δ 1)/2 > 0.5 is the observed synchrotron spectral index). Consequently, the particle energy scales as E δ+2 min, where E min is the lower bound of the power law energy distribution. Minimum Energy of Leptons in Radio Galaxies and Quasars The low energy end of the spectrum of the relativistic particles in classical radio sources is essentially unconstrained. Shocks which form at the head of the jets (hot spots) in radio sources are the responsible of most of the acceleration process. The spectrum of electrons accelerated in these regions and transported trough the radio emitting volume is a power law in momentum with a break and a high energy cut off (e.g., reviews by Leahy 1991; Eilek 1991; Brunetti 2004b). The diffusive shock acceleration model which is commonly adopted here (e.g., Heavens & Meisenheimer 1987; Blandford & Eichler 1987) provides a good description of the physics for particles with a Larmor gyroradius larger than that of the thermal protons (which give the thickness of the shock). This corresponds roughly to a Lorentz factor 23

24 of the relativistic electrons (and positrons) of γ Below this limit the physics of shock acceleration becomes much complex and depends on the shock structure and eventually on the particle pre acceleration mechanisms at work (e.g., Eilek 1991; Hoshino et al. 1992). What happens below γ is thus difficult to say and sometime - conservatively - people assume a flattening or a low energy cut off in the spectrum at these energies. An additional complications comes in the case of particle acceleration at relativistic shocks which may form along the jets of radio galaxies. These shocks produce a power law of accelerated particles with a well defined slope δ (e.g., Ellison, Reynolds & Jones, 1990; Kirk et al. 2000; Achterberg et al. 2001). In this case, still assuming diffusive acceleration theory, an additional constrain to the minimum energy of the bulk of the accelerated particles comes from the energy gain of particles at the first encounter with the shock E min Γ 2 (Vietri 1995, Achterberg et al. 2001), Γ the Lorentz factor of the shock. LOFAR LOFAR is expected to provide a major step in our understanding of the physics of radio sources. Here I focus on the study of the low energy part of the spectrum of the emitting leptons. The energy of leptons emitting at frequency ν o in a magnetic field B is E (nu o /B) 1/2. Present studies of radio galaxies are essentially carried out at GHz frequencies and may constrain the slope of the γ emitting electrons. LOFAR will reach unprecedented sensitivities and resolution in the frequency range MHz and this will immediately allow to study the spectrum of γ 1000 electrons. This is an appreciable step forward, however this is still not enough to catch the bulk of the particle energy in radio sources and to constrain the physics of particle acceleration in these sources. In addition, it should be noticed that we already have constraints on the spectrum of electrons at these energies from the studies of IC/CMB emission from the lobes of radio galaxies and quasars in the X ray band (e.g., Isobe et al. 2002; Comastri et al. 2003; Croston et al. 2005). LOFAR with Long-Baselines In order study the spectrum of electrons at γ << 10 3 with radio observations at 100 MHz, it is necessary to study regions with B > 100µG, i.e. powerful Hot spots and CSS. On the other hand these sources are compact, with typical apparent extension below 1 arcsec. Thus the combination of good sensitivity at low frequencies and of sub arcsec resolution is a unique possibility to constrain the low energy end of the electron spectrum in these sources km baselines are necessary to reach sub arcsec resolution at low frequency. Limits and Radio Hot Spots The observational strategy should account however for the limitations arising from the fact that sources should be observed in the optically thin regime. Indeed synchrotron self absorption gets an upper bound to the synchrotron emissivity ɛ of a optically thin source of size l : 24

25 l Const (1 + z) 9 2 ɛ 7+2α 2(3+α) ν e (4.1) This limits the possibility to study the electron spectrum at very low energies, since the energy of the emitting electrons at a given frequency scales with ɛ (assuming equipartition) : ( 1 γ ν e ) 1/2 ɛ 1 2(3+α) ν e (4.2) However, it can be shown that the synchrotron self absorption frequency in the case of compact Radio Hot Spots should be at ν < 50MHz and thus a large number of these sources are in the optically thin regime within the LOFAR frequency range. Conclusion The still unexplored low energy end (hopefully at γ 100) of the spectrum of the emitting electrons contains crucial information for the physics of radio sources. In order to study the emission from these electrons spatially resolved observations of compact radio sources at 100 MHz are necessary. LOFAR with 1000 km baseline will reach the necessary spatial resolution and sensitivity to obtain an unprecedented result : the measure of the synchrotron emission from γ electrons in Radio Hot Spots (and hopefully CSS). References [1] Achterberg A., Gallant Y.A., Kirk J.G., Guthmann A.W., 2001, MNRAS 328, 393 [2] Brunetti, G., 2003, in Matter and Energy in Clusters of Galaxis, ASP Conf. Series, vol.301, p.349, eds. S. Bowyer and C.-Y. Hwang [3] Brunetti, G., 2004a, JKAS 37, 493 [4] Brunetti, G., 2004b, in The Role of VLBI in Astrophysics, Astrometry and Geodesy. NATO Series II, mathematics, physics and chemistry, Vol. 135, F.Mantovani, A.Kus eds. Kluwer Academic Publishers, Dordrecht, The Netherlands, 2004, p.29 [5] Comastri A., Brunetti, G., Dallacasa D., Bondi M., Pedani M., Setti G., 2003, MNRAS 340L, 52 [6] Croston J.H., Hardcastle M.J., Harris D.E., Belsole E., Birkinshaw M., Worrall D.M., 2005, ApJ 626, 733 [7] Blandford, R. D., Eichler, D., 1987, Physics Rep., 154, 1 [8] Ellison D.C., Reynolds S.P., Jones F.C., 1990, ApJ 360, 702 [9] Heavens A.F., Meisenheimer K., 1987, MNRAS 225, 335 [10] Hoshino M., Arons J., Gallant Y.A., Langdon A.B., 1992, ApJ 390, 454 [11] Isobe N., Tashiro M., Makishima K., et al., 2002, ApJ 580L, 111 [12] Kirk J.G., Guthmann A.W., Gallant Y.A., Achterberg A., 2000, ApJ 542, 235 [13] Lazarian A., 2006, AN 327,

26 [14] Leahy J.P., 1991, in Beams and Jets in Astrophysics, eds. P.A.Hughes (Cambridge astrophysics series), p.428 [15] Eilek J.A., Hughes P.A., 1991, in Beams and Jets in Astrophysics, eds. P.A.Hughes (Cambridge astrophysics series), p.100 [16] Meisenheimer K., 2003, New Astr. Rev. 47, 495 [17] Sarazin C., 2002, in Merging Processes in Clusters of Galaxies, vol.272, p.1, edited by L. Feretti, I. M. Gioia, and G. Giovannini. [18] Vietri M., 1995, ApJ 453, Low-luminosity extragalactic radio jets with extended LOFAR Robert Laing, ESO, Garching, Germany Jet composition and the low-energy electron spectrum One of the most important unanswered questions in jet physics is that of composition: relativistic electrons and magnetic field must be present, but what are the relative energetic contributions of positrons, protons (cold or relativistic) and Poynting flux? There is no reason to suppose that jets have the same composition on all physical scales, and low-power (FR I) jets must entrain significant amounts of thermal plasma in order to decelerate. Methods for estimating jet composition often compare the energy flux (estimated from lobe dynamics, most reliably using observations of cavities in the surrounding plasma; e.g. Birzan et al. 2004) with constraints on the relativistic particle and field content from the observed radiation (e.g. Reynolds et al. 1996). A related technique, developed by Laing & Bridle (2002) for 3C 31, uses the mass flux. Both methods are limited by lack of knowledge of the low-energy part of the relativistic electron energy spectrum: this makes it impossible to decide whether the positive charges are protons or positrons. Typical Lorentz factors of radiating electrons in the jet bases we have observed at GHz are 10 4, assuming equipartition magnetic fields. In order to set interesting constraints on the composition, we need to find out whether the energy spectrum flattens for γ < 1000 (some theoretical arguments suggest a turnover below γ m p /m e = 1836). If so, much of the jet mass and energy flux must be carried by protons. Since γ ν 1/2, where ν is the observing frequency, LOFAR at 30 MHz would be able to study electrons with γ 800. The resolutions required are typically 5 arcsec (our observations with the VLA and the Pie Town VLBA antenna at 74 MHz are not adequate to resolve the jet bases). Particle acceleration and loss processes: spectra over a wide frequency range The traditional picture in which relativistic electrons in FR I jets are accelerated close to the nucleus and lose energy monotonically by synchrotron, inverse Compton and adiabatic processes is being challenged by recent high-resolution observations. It is now clear that electrons must be locally re-accelerated to very high Lorentz factors (γ 10 8 ) within a few kpc of the nucleus in order to generate the observed synchrotron X-rays (e.g. Hardcastle et al. 2002). We have recently established that the jets in NGC 315 show transverse spectral gradients 26

27 in these regions, probably associate with velocity shear (Laing et al. 2006; Figure 4.2). Increasing the frequency range at the resolution of these observations ( arcsec) from the current GHz is essential to understanding the origin of these gradients (e.g. whether the spectra power are laws, or show evidence for energy-dependent losses) and their relation to the X-ray emission (a) 5.5 arcsec DECLINATION (J2000) RIGHT ASCENSION (J2000) (b) 1.5 arcsec 40 DECLINATION (J2000) RIGHT ASCENSION (J2000) Figure 4.2: Spectral-index variations in the jets of NGC 315 between 1.4 and 5 GHz (Laing et al. 2006). Note the flattening with distance from the nucleus and towards the edge of the jet, in regions where we infer velocity shear. The spectral structure of the extended lobes is also complex, with flatspectrum jets appearing to propagate within steeper-spectrum lobe emission (Katz-Stone & Rudnick 1997; Katz-Stone et al. 1999; Figure 4.3). Most highresolution spectra cover no more than a decade in frequency and cannot distinguish between different models of electron evolution. Extended LOFAR has a key role to play in determining the low-frequency spectra. Comparison with EVLA and single-dish observations at higher frequencies will allow us to image spectral variations over a factor of 1000 in frequency and therefore to determine spectral curvature with unprecedented accuracy. The magnetoionic environment: low Faraday depths In our investigations of Faraday rotation and depolarisation in FR I radio galaxies, it has become clear that extremely low Faraday depths may sometimes 27

28 DECLINATION (J2000) DECLINATION (J2000) RIGHT ASCENSION (J2000) RIGHT ASCENSION (J2000) RIGHT ASCENSION (J2000) Figure 4.3: VLA observations of the nearby radio galaxy 3C 31 (data from Laing et al., in preparation). Left: the source structure in total intensity at 1.4 GHz; middle: spectral index. Right: Sobel-filtered total intensity. The resolution is 5.5 arcsec. be present. The clearest case is NGC 315 (Laing et al. 2006), where we detect rotation measure fluctuations of a few rad m 2 on scales of arcsec (3 30 kpc). We suppose that these are due to fields in the very tenuous medium associated with the surrounding galaxy group, but there is so little rotation of position angle across the VLA bands that we cannot be sure of this. Polarisation observations with LOFAR at resolutions of 1 5 arcsec at frequencies around 200 MHz would allow us to determine unambiguously whether the fluctuations are due to foreground material and also to investigate hints of a systematic gradient in rotation measure across the jets which could just possibly arise from an associated toroidal magnetic field. No unambiguous signature of thermal matter co-spatial with synchrotronemitting plasma have ever been detected by observations of linear polarisation, so we have only (rather poor) constraints on the internal density. The reasons are that the densities are likely to be very low, and observations have been confused by foreground Faraday screens (cf. above). LOFAR will finally give us the ability to search for very small amounts of thermal matter in the faint, steep spectrum regions of FR I radio sources at large distances from the host galaxy. This minimises the contamination from local foregrounds (which have typical scales 1 kpc and are therefore resolved by LOFAR) in parts of the sources where significant entrainment is thought to occur. LOFAR should be sensitive to Faraday depths 0.1 rad m 2, close to those predicted even for the inner regions of radio jets (Laing & Bridle 2002). References [1] Birzan, L., Rafferty, D. A., McNamara, B. R., Wise, M. W., Nulsen, P. E. J.: 2004, ApJ 607,

29 [2] Hardcastle, M. J., Worrall, D. M., Birkinshaw, M., Laing, R. A., Bridle, A. H.: 2002, MNRAS 334, 182 [3] Katz-Stone D.M., Rudnick L.: 1997, ApJ 488, 146 [4] Katz-Stone D.M., Rudnick L., Butenhoff C., O Donoghue A.A.: 1999, ApJ 516, 716 [5] Laing, R.A., Canvin, J.R., Cotton, W.D., Bridle, A.H.: 2006, MNRAS 368, 48 [6] Laing, R.A., Bridle, A.H.: 2002, MNRAS 336, 1161 [7] Reynolds, C.S., Fabian, A.C., Celotti, A., Rees, M.J.: 1996, MNRAS 283,

30 5 The high redshift Universe 5.1 Extragalactic surveys with LOFAR long baselines Matt J. Jarvis, Oxford, UK For many years, radio surveys have been known for their ability to find the rarest most powerful active galactic nuclei (AGN) over the history of the Universe (e.g. Longair 1966). The low-frequency surveys conducted at Cambridge over the past four decades have provided a wealth of information on these phenomena, including constraints on the evolution of black-hole accretion activity in the Universe (e.g. Longair 1966; Willott et al. 2001; Jarvis et al. 2001a), acting as beacons to the most massive galaxies at all redshifts (e.g. Lilly & Longair 1982; Eales et al. 1997; Jarvis et al. 2001b; De Breuck et al. 2002; Willott et al. 2003), and more recently using these powerful sources to pin-point protoclusters at the highest redshifts (Venemans et al. 2002, 2004; Overzier et al. 2006). As useful as such studies are in this field of research, the next generation of radio surveys conducted with the new radio telescopes will break into new parameter space for extragalactic surveys. Due to the massive increase in sensitivity of LOFAR over previous telescopes, the most prominent extragalactic sources will no longer be the AGN, but starburst galaxies. Using the radio-luminosity functions derived from previous low-frequency surveys for the more powerful radio sources populations such as the FRI and FRII (Fanaroff & Riley 1974) radio galaxies, along with a prescription for the radio luminosity of radio-quiet quasars derived from the X-ray luminosity function of Ueda et al. (2003), we are able to estimate the contribution of the AGN to any LOFAR survey. This follows the work described in Jarvis & Rawlings (2004) for the Square Kilometre Array (SKA). Furthermore, we are also able to estimate the contribution of the total source population from the star-forming population using the luminosity function from Yun, Reddy & Condon (2001) along with an assumed evolution which ensures that the source counts at both mid- and far-infrared wavelengths are not exceeded (see Blain et al. 1999). Assuming that a deep LOFAR survey could reach a rms sensitivity of 6 µjy at 200 MHz, this would imply a total sources density of around 20,000 per square degree. As shown in Figure 5.1, the vast majority of these sources will be starforming galaxies (about 80 per cent of the sources will be star-forming galaxies). Obviously, with this source density confusion becomes a major issue. This is particularly true with the current Netherlands-only LOFAR, where the spatial resolution is 3 4 arcsec. However, with both UK and German long baselines, LOFAR would be able to probe to much deeper fluxes. This would then allow unprecedented limits on the evolution of radio sources over the history of the Universe, and a depth that would probably not be feasible with other telescope over such large areas of sky until the SKA is on-sky. References [1] Blain, A.W., Smail, I., Ivison, R.J., Kneib, J.-P.: 1999, MNRAS 302, 632 [2] De Breuck, C., et al.: 2002, AJ 123,

31 Figure 5.1: The number of sources per square degree in a hypothetical deep LOFAR survey with S 200MHz > 30µJy. The solid line represents the star-forming galaxies, the dot-dashed line represents the radio-quiet quasars, the dashed line are FRI radio galaxies and the dotted line denotes the FRII radio galaxies. One can see that such a survey is dominated by the star-forming galaxies. [3] Eales, S., Rawlings, S., Law-Green, D., Cotter, G., Lacy, M.: 1997, MNRAS [4] Fanaroff, B.L. & Riley, J.M.: 1974, MNRAS [5] Jarvis, M.J., et al.: 2001a, MNRAS [6] Jarvis, M.J., et al.: 2001b, MNRAS [7] Jarvis, M.J. & Rawlings, S.: 2004, NewAR [8] Lilly, S.J. & Longair, M.S.: 1982, MNRAS [9] Longair, M.S.: 1966, MNRAS [10] Overzier, R.A., et al.: 2006, ApJ 637, 58 [11] Ueda, Y., Akiyama, M., Ohta, K., Miyaji, T.: 2003, ApJ [12] Venemans, B.P., et al.: 2002, ApJ [13] Venemans, B.P., et al.: 2004, A&A [14] Willott, C.J., Rawlings, S., Blundell, K.M., Lacy, M., Eales, S.A.: 2001, MNRAS [15] Willott, C.J., Rawlings, S., Jarvis, M.J., Blundell, K.M.: 2003, MNRAS [16] Yun, M.S., Reddy, N.A., Condon, J.J.: 2001, ApJ

32 5.2 Deep fields and high redshift star-forming galaxies R. J. Beswick, Jodrell Bank Obseratory, The University of Manchester, Lower Withington, Nr Macclesfield, Cheshire, SK11 9DL, UK Deep Fields - An introduction Over the last few years, some of the most impressive highlights of radio interferometry have come about from deep observations of discrete patches of sky. For example, recent MERLIN and VLA observations of the Hubble Deep Field North (HDF-N) and flanking fields (Richards et al. 1998; Muxlow et al. 2005) and the Australia Telescope observations of the Hubble Deep Field South (Norris et al. 2005; Nuynh et al. 2005) have begun to reveal and characterise the content of the microjansky radio sky. Complimenting the success of the optical studies of both the HDF-N and HDF-S (Williams et al. 1996, 2000) have lead to significant advances in our understanding of the Universe, deep radio studies have provided an important observational niche, since they probe physical conditions that are faint or invisible at optical and infrared wavelengths plus importantly, they are unaffected by dust extinction. These radio HDF-N observations (Muxlow et al. 2005) have not only astrometrically calibrated the HDF, allowing observations at other wavebands to be reliably correlated but more importantly enabled the starformation rate (SFR) as a function of redshift to be established to z =1.5 (and tentatively to z 4). By virtue of the high degree of astrometric alignment of deep radio images with observations made at other wavelengths it is possible to use the large number of catalogued sources in different wavelength bands to statistically detect the radio counterparts of optical sources down to hitherto unobtainable sensitivity limits. Such studies offer a glimpse of the micro and even sub-microjansky radio sky thus allowing us to begin to predict the wealth of sources that the next generation of radio interferometers, such as LOFAR, will detect. The microjansky radio source population The sources and their sizes At 1.4 GHz the vast majority (70 per cent) of radio sources below 100µJy with the HDF-N are identified as starburst or composite starburst systems (Muxlow et al. 2005). Other radio studies such as those by Hopkins et al. (1998), Norris et al (2005) and Huynh et al (2005), also show that the population of faint radio sources is dominated by emission from radio starbursts. Source counts and confusion Statistically it is already possible to detect the microjansky source population in radio deep fields. Using deep MERLIN and VLA observations at 1.4 GHz an 8.5 area centred upon the Hubble Deep Field North has been imaged (Muxlow et al. 2005; 2006 in prep; Thrall et al., 2006 in prep). Within this field Muxlow et al (2005), detected 92 radio sources in this field to a conservative detection threshold of 40 µjy. However, galaxies are detected in deep HST ACS z-band images of this area. Whilst the vast majority of these sources are not individually detected at radio wavelengths it is possible to statistically 32

33 detect these faint optical systems in the radio. Figure 5.2 (left) shows the binned radio flux of 8000 of these sources (excluding radio sources >20µJy and all potentially confused sources) versus their optical brightness. Optical sources as faint as 25th magnitude are statistically detected at the level of a few micro- Jansky and is confirmed in a stacked average image of the radio emission from 1000, 23rd magnitude galaxies also shown in Figure 5.2. This implies that approximately half of the 2700 galaxies brighter than 24th magnitude will have radio flux densities of greater than 4 µjy at 1.4 GHz within this field. Assuming that the radio emission arising from this faint radio source population is related to star-formation with a radio spectral index 0.7, a LOFAR survey that has a 5σ detection limit of 20µJy at 200 MHz will detect many tens of thousands of normal star-forming galaxies per square degree. 1 It should also be noted that this value is a lower limit on the number of detections since several other deep radio surveys (e.g. VIRMOS, Phoenix Deep Field and the ATCA HDF-S observations) have found systematically higher source counts than identified in the HDF-N, presumably due to cosmic variance. These faint, predominantly star-forming, radio sources will dominate the sub-mjy radio sky observed by LOFAR. 1.5 Average emission from 957 galaxies in the GOODS-N field Z-band sources between magnitudes ARC SEC ARC SEC Figure 5.2: Left: Statistical detection of the µjy radio emission from >8000 optically selected galaxies within an 8.5 square arcminute field centred on the HDF-N (excluding all radio emission brighter than 20µJy bm 1 ). Radio emission is statistically detected from galaxies brighter than magnitude 25. The red points represent a control sample of randomly selected positions away from any catalogued source. Right: The contoured image shows the average radio (contour) and z-band optical emission from 957 galaxies in the HDF-N with magnitudes between The contours are 2 times 340 njy beam 1. (Figures from Muxlow, Thrall, Beswick et al. in prep). Within the scope of the presently funded LOFAR project, the longest LO- FAR baselines will be of order 100 km, resulting in an angular resolution of Although the sensitivity of the array is not significantly compromised by the de-scoping of the original LOFAR project the lack of longer, pan-european baselines significantly compromises the angular resolution of the array. Deep field observations with this limited resolution will quickly become confusion limited with the current array. As such it is essential that the baseline length 1 In order to reach these sensitivity levels very large dynamic ranges will need to be achieved. 33

34 and hence resolution of LOFAR be increase if deep field observations are not to be confusion limited and are to reach their scientific potential. Resolving high redshift galaxies In addition to inferring the number of sources that the next generation of radio interferometers, such as LOFAR, e-merlin, the evla and the SKA may observe, current statistical studies of high resolution deep field observations also provide a glimpse of the structures and sizes of these sources. By using similar methods to those described above the brightness profiles of faint radio sources associated with groups of catalogued optical sources can be derived. Shown in Figure 5.3 are two sets of radial radio profiles of sources in the HDF- N. To the left the radial profiles of three typical bright (few tens of µjy) radio sources are shown. Of the 92 (>40 µjy bm 1 ) radio sources detected by Muxlow et al. (2005) virtually all were resolved, with angular sizes of between 0. 2 and 3. By statistically combining the radio emission from the positions of optically detected galaxies the radial radio profiles of these optical galaxies can be constructed. As can be seen in Figure 5.3 (right) the radial profiles of the faint radio emission from optical galaxies, typically imply source sizes of up to arcsec ( kpc at z 2). As may be expected the angular size of sources gradually reduce for ever fainter galaxies that are presumably, on average, more distant sources. Figure 5.3: Radial profiles of radio sources within the HDF-N. Left: radio radial profiles of three bright radio starburst galaxies in the HDF-N. The dashed vertical lines represent the 2-D Gaussian fitted radii of these sources. Right: composite radial radio profiles of the optically detected sources with the HDF-N within individual optical magnitude bins. As can be seen in Figure 5.4, the sub-arcsecond angular resolution of the MERLIN+VLA observations of the HDF-N are ideal for imaging these sources. Without baselines longer than 100 km the vast majority of high redshift starforming galaxies detected by LOFAR will be unresolved points. Whilst merely detecting vast numbers of these galaxies will undoubtedly be of significant value, only with the longer international baselines providing sub-arcsecond imaging capabilities at the highest LOFAR frequencies will it be possible to begin to image the complex structures of these faint radio sources. Identifying the high redshift radio sources detected by LOFAR at other wavelengths 34

35 J Starburst, z= DECLINATION (J2000) 15 DECLINATION (J2000) J Wide-Angle Tail RIGHT ASCENSION (J2000) RIGHT ASCENSION (J2000) Figure 5.4: Images of two galaxies within the HDF-N, one starburst galaxy and one wide-angled tail radio galaxy. In both images, 1.4 GHz contours are shown overlaid upon a 4-colour (BVIz) HST ACS image. The radio images have been contoured at 2 times 10 µjy beam 1 (Muxlow et al. 2005). Increasingly it is being recognised that to understand astrophysical phenomena a pan-chromatic view of the Universe is required. LOFAR will uniquely contribute to this view by providing high-sensitivity observations of the littleexplored low frequency Universe. In order that the science from deep LOFAR observations can be complimented by studies at other wavelengths using both current and the next generation of astronomical instruments (e.g. e-merlin, EVLA, evlbi, ALMA, NGST, VLTI, Spitzer, Herschel, Chandra, Swift etc) it is essential that LOFAR observations have comparable angular resolutions to these other instruments (i.e. ideally < 1arcsec) and also that they can be astrometrically aligned with sub-arcsecond precision. Clearly the need for LOFAR to provide low radio frequency images at angular resolutions complimentary to those provided by other contemporary instruments is a science driver for Extended LOFAR baselines and sub-arcsecond imaging capabilities. The necessity for accurate astrometry can be easily highlighted by the case of the sub-mm sources identified by SCUBA. For example, only by using very accurate absolute positions from radio observations was it possible to identify the brightest sub-mm source with the HDF-N (HDF850.1, Muxlow et al. 2005; Dunlop et al. 2004) with an optically faint and extremely red, redshift 4 galaxy situated within 100 of several brighter, nearby elliptical galaxies. Increasingly, as higher sensitivity observations are made at all wavelengths and fields become more crowded with sources it is essential that observations be able to both separate and correctly identify sources at different wavelengths. Extending the IR-RADIO correlation to lower radio frequencies and luminosities Over 25 years ago it became clear that the radio and global infrared emission from galaxies was tightly correlated (van der Kruit et al. 1973; Condon et al. 35

36 1982). In the 1980s data from the IRAS satellite demonstrated that this correlation extended over many orders of magnitude from nearby dust-rich dwarfs to ultra-luminous infrared galaxies (ULIRGs). More recently comparing observations from NASA s Spitzer IR satellite and radio observations has extended this correlation (Appleton et al. 2004; Beswick et al in prep) both to the mid-ir and over a still wide range of luminosities (see Figure 5.5). This correlation can be extended to even fainter luminosities (L 25 µm W Hz 1 ) by considering discrete regions within individual nearby star-forming galaxies (Murphy et al. 2006). The correlation between the radio and infrared emission arises because both are related to the star formation processes; the infrared emission is produced from dust heated by photons from young stars, while the radio emission arises from synchrotron radiation produced by the acceleration of charged particles from supernovae explosions. Figure 5.5: 20 cm radio and 24 µm IR luminosity correlation for 24 µm selected Spitzer sources within the HDF-N. Luminosities have been k-corrected in the IR assuming a SED slope equivalent to Arp220 and in the radio assuming a (1+z) 0.7 boosting. (Figures from Beswick, Muxlow, Thrall et al. in prep). While recent deep field observations in the radio (using MERLIN, VLA & the WSRT) and IR (primarily using Spitzer) have made significant advances in understanding the relationship between decimetre radio emission and mid-ir emission in star-forming galaxies (both nearby galaxies and out to z 4 (e.g. Garrett 2002)), the relationship between the emission in IR and at low radio frequencies is still unexplored. LOFAR will explore the lower frequency ( MHz) radio emission from millions of star-forming galaxies and, in conjunction with the current generation of IR satellites such as Spitzer, will extend the radio-ir correlation to lower radio frequencies and luminosities, allowing the investigation of any evolution of this relationship with redshift, luminosity and radio frequency. Whilst surveys using LOFAR with 100 km baselines will be able to detect a large number of sources and will contribute to this work, as discussed earlier, the lack of resolution will result in the confusion limit being quickly reached and hence significantly reducing both the number and luminosity range of sources 36

37 that can be observed. Summary The need for Extended LOFAR The planned Extended LOFAR array consisting of the dutch-based LOFAR core plus outlying stations situated in countries such as Germany, Sweden, France, Italy and the UK, will extend the maximum baseline lengths of the array to several hundred and possibly 1000 km. This extension will result in high sensitivity, sub-arcsecond angular resolution observations being achievable with LOFAR. As outlined in each of the sections above this will be essential to the success of intermediate and deep surveys made using LOFAR. References [1] Appleton et al. 2004, ApJS, 154, 147 [2] Beswick et al. 2006, in prep [3] Dunlop et al. 2004, MNRAS, 350, 769 [4] Garrett 2002, A&A, 383, L19 [5] Huynh et al. 2005, AJ, 130, 1373 [6] Murphy et al. 2006, ApJ, 638, 157 [7] Muxlow et al. 2005, MNRAS, 358, 1159 [8] Muxlow et al. 2006, in prep [9] Norris et al. 2005, AJ, 130, 1358 [10] Richards et al. 1998, AJ, 116, 1039 [11] Thrall et al in prep [12] Williams et al AJ, 1996, 1335 [13] Williams et al. 2000, AJ, 120, Ultra-steep spectrum sources with an extended LO- FAR Probing the most distant massive galaxies, proto-clusters and massive black holes George Miley, Huub Röttgering, Sterrewacht Leiden, Netherlands Introduction Distant luminous radio galaxies are unique probes of the early Universe (e.g. Miley 2000) because (i) they are the progenitors of most massive galaxies (dominant cluster galaxies), (e.g. Pentericci et al. 1997) (ii) their nuclei contain massive black holes, (iii) they pinpoint the progenitors of rich galaxy clusters (Pentericci et al. 2000, Venemans et al. 2002, 2005, Miley et al. 2004) and (iv) at z > 6 they are potential diagnostics of reionisation, via redshifted HI absorption. These objects are therefore unique test beds for investigating many fundamental questions in cosmology, such as relative timescales of formation of the different components of galaxies. Also, because they are amongst the most 37

38 luminous known objects, their radio emission provides an important laboratory for studying high-energy physics. LOFAR is well suited for detecting the most distant luminous radio galaxies. The most efficient method for finding distant radio galaxies uses an empirical correlation between radio spectral steepness and distance (Blumenthal and Miley 1979). Optical spectroscopic follow-up of the steepest ten percentile of luminous radio sources (ultra-steep spectrum sources, spectral index, α -1) has proved a successful strategy for finding high-redshift radio galaxies. More than 300 z > 2 luminous radio sources are now known (Miley and de Breuck, 2007) and most of them have been found using the z α correlation. To date the most distant radio galaxy has a redshift of z 5.2. Because LOFAR will survey the sky to unprecedented depth at low-frequencies, LOFAR will be sensitive to the steepest spectrum radio sources and LOFAR surveys should contain even more distant luminous radio galaxies. Here it will be shown that for a 100km LOFAR such a project is limited by confusion and that extending the baseline of LOFAR beyond the presently planned 100 km will be extremely beneficial for such searches. ~ z CORRELATION CONVENTIONAL EXPLANATION Concave radio spectrum e.g.cygnus A Figure 5.6: Left: shows spectral index-redshift relation for luminous radio sources (e.g. De Breuck et al. 2002, Blumenthal and Miley, 1979). This is often explained in terms of a radio K-correction. A typical spectrum of an extended radio source is concave, with a gradual steepening over the relevant frequency range, that is usually attributed to radiation losses. For a radio source observed at a fixed frequency, the rest frequency sampled is higher if the source has a larger redshift. If the radio spectrum is concave, a steeper part of the rest-frame spectrum is sampled for higher redshift sources. Producing the z α correlation 38

39 To develop the optimum strategy for finding such objects with LOFAR it is necessary to consider the origin of the z α correlation. The conventional explanation for the z α correlation is that it is the result of a concave radio spectrum coupled with a radio K-correction (see Figure 5.6). If the redshift of the object is larger, observations at a fixed survey frequency sample higher rest-frame frequencies where the spectrum is steeper. However, there is increasing evidence that such an explanation is not the correct one, because the radio spectra of distant luminous radio galaxies are not concave at the relevant frequencies. More than 15 years ago an accurate spectrum of 4C41.17, a distant radio galaxy Rosetta stones at z = 3.8, was presented by Chambers, Miley and van Breugel (1990). It was found to have an extremely straight spectrum at frequencies between > 40 MHz and 4.7 GHz, the relevant frequency range for the z α correlation. Although the spectrum steepens above 5 GHz, presumably through radiation losses, this is too high a frequency to play a role in the z α correlation. Furthermore, a recent study by Klamer et al. (2006) showed that 33 of 37 sources in their SUMSS-NVSS sample have straight spectrum between 0.8 and 18 GHz. The data therefore indicate that the spectrum of distant radio galaxies are generally not concave over the relevant frequency range. An additional explanation of the z α correlation is needed. Several additional possibilities have been proposed to explain the z α effect: (i) The z α is an indirect manifestation of a luminosity α effect (Chambers et al. 1990, Blundell and Rawlings, 1999). The straightness of the 4C41.17 spectrum (Figure 5.7) indicates that radiation losses do not dominate below 5 GHz. Classical synchrotron theory predicts that a simple continuous particle injection model will result in a spectrum with a low-frequency cut-off whose frequency depends on the source luminosity, L 6/7. At high luminosities (preferentially large z) the frequency of this cut-off decreases. At the relevant frequencies, this would result in a L α effect. Through Malmquist bias, this would be observed as a z α effect. However, an argument against this explanation was provided by Athreya and Kapahi (1998). They showed that a z α correlation existed in samples with a limited range of L, indicating that the L α explanation is incorrect, or at least not the whole story. (ii) There is a physical effect that causes the spectral index to steepen with higher ambient density. Athreya and Kapahi (1998) proposed that such a mechanism exists. A dense environment would cause the upstream fluid velocity to decrease and this would result in a steeper spectrum in a first-order Fermi acceleration process. Recently Klamer (2006) pointed out that such a mechanism would result in both a z α correlation and a L α correlation and would provide a natural physical link between high redshift radio galaxies and nearby cluster halos, both of which are observed to have ultra-steep spectra. However, in general the density of the medium around high-z radio galaxies is highly nonuniform, but the spatial variation of spectral index across the source is small. Together, this provides a strong argument against the validity of such an explanation. The internal dispersion of spectral index across spatially-extended high-z radio sources is much less than the source-to-source dispersion indicated by the integrated spectral indices of a sample (e.g. Carilli et al. 1997). If the ambient medium is highly non-uniform, how can the right side of a radio source know that the left side has an uncommonly steep spectrum? My conclusion is that the nature of the z α effect is still unclear and 39

40 RADIO SPECTRUM OF 4C 41.17, z = 3.8 Figure 5.7: The radio spectrum of 4C41.17 from Chambers, Miley and van Breugel (1990). Shown is both the integrated spectrum of the whole source and the spectra of the components. The spectral shape is ultra-steep and straight in the range of < 40 MHz up to 8 GHz. The spectrum is not concave and therefore not consistent with a K correction explanation for the z α correlation. According to classical synchrotron theory the dominant straight region of the spectrum is indicative of the regime where radiation losses are balanced by particle injection. Radiation losses dominate above 5 GHz and a low-frequency cut-off due to synchrotron absorption below 40 MHz. Both radio source components have ultra-steep spectra. Straight ultra-steep spectra with relatively small spectral variation are a general property of high-redshift luminous radio galaxies. that a straight ultra-steep spectrum must be produced by a mechanism within the galaxy nucleus and is not an in-situ effect determined by the (highly nonuniform) extranuclear environment. Lessons for LOFAR distant radio source searches The empirical existence of the z α effect and the empirical demonstration that the spectrum of 4C41.17 is straight at frequencies > 40 MHz has consequences for optimising distant radio galaxy searches. It should be aimed for a survey with a low enough observing frequency that a low-frequency cut-off in the spectra of z 4 sources can be measured. The presence or not of curvature in the spectra could then be used to discriminate between sources at z < 5 and sources at z > 5. Figure 5.7 indicates that 30 MHz may not be sufficiently low a survey frequency to optimally detect the most distant luminous radio galaxies. Hence, it is imperative that LOFAR be developed to conduct the deepest possible surveys at 15 MHz. 40

41 Presently surveys at 30 MHz and below are confusion-limited. A long-baseline extension of LOFAR at 15 MHz is crucial to reducing confusion and opening up new parameter space and detecting the most distant radio galaxies possible. A 400km LOFAR would reduce nominal confusion by almost an order of magnitude. To minimise the effects of the ionosphere and RFI, observations at 15 MHz will probably have to be repeated several times under the best possible conditions. There are other aspects of distant luminous radio galaxies that 15 MHz observations with an extended baseline LOFAR would facilitate besides the main goal specified here of searching for z > 5 radio galaxies. (i) The statistics of the steepest spectra sources (spatial clustering and luminosity functions) will provide new constraints on AGN evolution and its relation to galaxy evolution in general. (ii) The spatial distributions of spectral index across radio sources in the MHz range will be important constraints of the physics of distant luminous radio sources and in particular of the nature of the spectral-index redshift effect. A baseline length of > 300 km is a minimum needed to provide the needed spatial resolution. Conclusions The possibility of detecting radio galaxies at distances larger than was hitherto possible is a fundamental driver for extending LOFAR to longer baselines. However, an even more exciting aspect of an extended LOFAR is that at 15 MHz it will open up a substantial region of unexplored parameter space (sensitivity and resolution). Such a facility would be therefore well placed to discover new types of objects and new phenomena. References [1] Athreya, R. M., & Kapahi, V. K. 1998: Journal of Astrophysics and Astronomy 19, 63 [2] Blumenthal, G., Miley, G., 1979: A&A 80, 13 [3] Blundell, K. M., & Rawlings, S., 1999: Nature 399, 330 [4] Carilli,C. L., Röttgering,H. J. A., van Ojik,R.,1997: ApJ Suppl. 109, 1 [5] Chambers, K. C., Miley, G. K., & van Breugel, W. 1990: ApJ 363, 21 [6] De Breuck, C., van Breugel, W., Stanford, S. A., Röttgering, H., Miley, G., & Stern, D. 2002: AJ 123, 637 [7] Klamer,I., PhD Thesis, 2006, University of Sydney [8] Miley, G. K. and De Breuck, C., 2007: Astronomy and Astrophysics Reviews, in preparation. [9] Miley, G. 2000: From Extrasolar Planets to Cosmology: The VLT Opening Symposium, Proceedings of the ESO Symposium held at Antofagasta, Chile, 1-4 March Edited by Jacqueline Bergeron and Alvio Renzini. Berlin: Springer-Verlag, p. 32., 32 [10] Miley, G. K., et al. 2004: Nature 427, 47 [11] Pentericci, L., Röttgering, H. J. A., Miley, G. K., Carilli, C. L., & McCarthy, P. 1997: A&A 326,

42 [12] Pentericci, L., et al. 2000: A&A 361, L25 [13] Venemans, B. P., et al.: 2002: ApJ Lett 569, L11 [14] Venemans, B. P., et al. 2005: A&A 431, LOFAR and associated H I in high-z radio sources Raffaella Morganti, ASTRON, Dwingeloo, and Kapteyn Institute, Groningen, Netherlands LOFAR will make it possible to study the presence of associated neutral atomic hydrogen observed in absorption against radio sources with redshift larger than 5. These observations are of particular interest because it is known that high-redshift radio sources (in particular radio galaxies) are often enshrouded in massive reservoirs of gas. The presence of a dense medium is indicated by the detection of CO. Few cases are now known (e.g. Klamer et al and ref. therein). Furthermore, these radio galaxies are often surrounded by large Lyα halos and they may pinpoint the location of rich protoclusters (Miley et al. 2004). The detection of H I in high-z radio galaxies has been, so far, not very successful. At z up to 0.8 many detections - often in compact and young radio sources (Vermeulen et al and ref. therein) - have been found. At z 3 only one detection is known so far: J (Briggs et al. 1993). Are these high-redshift radio source really devoid of neutral atomic gas? LOFAR will allow to investigate this thanks to: i - Possibility of removing RFI in a very efficient way thanks to the large number of channels available. ii - High sensitivity. If one assumes a sensitivity of the full array between 0.06 and 0.07 mjy/b (in continuum, in the range MHz for 1 h of data over 4 MHz bandwidth) this will result in 0.15 mjy/b/ch noise for 40h of data over a 10 km/s wide channel. For a radio source of 100 mjy this will mean to be able to detect (at 3σ) an H I absorption with τ = 0.005, comparable to the weaker absorptions detected in nearby radio sources. In addition to this, one of the LOFAR key projects (Surveys of the Radio Sky, PI H. Röttgering) will be looking for high redshift radio sources and it will produce a database of interesting targets for follow up H I observations. Why the need for high spatial resolution? High redshift radio sources are known to have a complex structure on the arcsec scale. The resolution of the standard LOFAR will be between 3 and 6 arcsec, thus not high enough to resolve the exact location of the H I absorption and, therefore, understand its origin. Observations of nearby radio galaxies have shown that the neutral hydrogen can be located in different structures: - Circumnuclear disks/tori - Clouds that can be a tracer of the rich and clumpy medium in which the radio source is embedded. This medium can affect the evolution of the radio source by momentarily destroy the path of the jet (see e.g. the case of 4C 12.50, Figure 5.8) - Fast outflows. Fast H I outflows have been detected in nearby radio galaxies (Morganti et al. 2005). The feedback they produce can be extremely important 42

43 Figure 5.8: Continuum (contours) and total intensity of the HI absorption (orange) in the radio source 4C12.50 (from Morganti et al. 2004). The absorption occurs 100 pc from the core. This cloud indicates the presence of a rich and clumpy interstellar medium in the centre, likely left over from the merger that triggered the activity and that this medium influences the growth of the radio source. The location of the cloud at the edge of the northern radio jet/lobe suggests that the radio jet might be interacting with a gas cloud. This interaction could be responsible for bending the young radio jet (Morganti et al. 2004). for the evolution of the host galaxy, up to the point that it could limit the growth of the nuclear black-hole. Identify the location of the H I outflows is crucial in order to understand their nature (e.g. interaction between the radio jet and the ISM). All these physical processes (in particular the effect of the ISM on the radio source evolution and the effect of outflows) are considered to be even more relevant for high redshift radio sources. The neutral hydrogen can, therefore, represent a powerful tracer and the high resolution study of this gas will help in understanding the conditions around AGN in the early Universe. References [1] Briggs et al. 1993, ApJ Letter 415, 99 [2] Klamer et al. 2005, ApJ Letter 621, 1 [3] Miley et al Nature 427, 47 [4] Morganti et al. 2004, A&A 424, 119 [5] Morganti et al A&A Letter 444, 9 [6] Vermeulen et al. 2003, A&A 401,

44 6 Strong gravitational lensing 6.1 Introduction Olaf Wucknitz, JIVE, Dwingeloo, NL Mike Garrett, JIVE, Dwingeloo, NL Neal Jackson, Jodrell Bank, UK Dieter Engels, Hamburger Sternwarte, Germany The gravitational lens effect has a multitude of astrophysical applications [21, 15]. It can be used as a natural telescope to study lensed background sources in great detail, it provides information about the lenses themselves and about the spacetime between source and observer (Hubble constant and cosmological parameters). In addition lensing produces multiple images that can be used to study extinction, scattering and other propagation effects. An important extragalactic application is the determination of mass distributions of lens galaxies and clusters in order to study their structure and evolution. Lensing is the only method that can provide accurate information for that purpose even for very distant galaxies. In contrast to all other methods, this information is independent of baryon content and light emission and constitutes a direct and unbiased measurement of the combined luminous and dark mass. Positions and relative magnifications of multiply lensed images of background sources are used as constraints for models of the lensing potential and thus the mass distribution. Compared to e.g. lensed QSOs, lensed extended sources offer much more information, because each of their components provides its own set of constraints. Several approaches to utilise this information have been developed and applied [23, 22, 16]. The role of LOFAR in gravitational lensing will be two-fold. The unique capabilities to survey large parts of the sky with good resolution and high sensitivity will be used to conduct the largest surveys for lensed radio sources so far, which will increase their number by an order of magnitude. High resolution provided by long baselines is essential to identify the promising lens candidates and to keep the number of candidates for follow-up observations in a feasible range new lenses found by LOFAR are realistic for 400 km baselines, compared to < 40 radio lenses known today. The potential number for significantly longer baselines is > The source counts associated with deep LOFAR surveys are expected to be dominated by a population of (largely) steep spectrum and cosmologically distant star forming galaxies. By using a foreground galaxy clusters as a giant magnifying glass, it will be possible to detect intrinsically faint star forming systems at very high redshift. The magnification will also permit the radio continuum (star formation) morphology of these sources to be studied in unprecedented detail. An extended LOFAR is essential for these observations, in order to resolve out the extended emission associated with the foreground cluster gas. 6.2 Lens surveys CLASS, the only large-scale survey for radio lenses conducted so far, was explicitly tailored to search for compact sources [18, 5]. In other projects, lensed radio lobes were searched [17, 12] based on the FIRST survey. However, both 44

45 the resolution and the source number of this survey proved insufficient to find a large number of lenses. LOFAR is the first radio telescope that can provide a large-scale radio survey with sufficient resolution and the large number of sources that is required to find a significant amount of new lenses. Table 6.1 gives an overview of planned LO- FAR surveys to show that a completely new parameter space in source number and resolution will be explored with these projects. A good fraction ( 50 %) of the LOFAR sources will have sizes of 1 2, mostly star-burst galaxies with substructure on all scales. This is exactly the class of sources that can probe lensing potentials most accurately. Table 6.1: Details of two LOFAR surveys that will be conducted in the coming years. The most important existing radio surveys are shown for comparison. LOFAR resolutions are estimated for 400 km baselines. The survey properties are based on an extrapolation from shorter baselines, assuming sufficient sensitivity on the long baselines. To be able to reach this goal, it is necessary to have as many long-baseline stations available for the surveys. survey frequency area source number resolution array rms/flux limit source density LOFAR-120 a 120 MHz half-sky /43 µjy /deg 2 LOFAR-200 b 200 MHz 250 deg /14 µjy /deg 2 FIRST c 1.4 GHz galactic caps VLA B 0.15/1 mjy deg 2 90/deg 2 NVSS d 1.4 GHz δ > VLA D/DnC 0.45/2.5 mjy 53/deg 2 WENSS+WISH e 330 MHz δ > WSRT f 4/18 mjy 26 < δ < 9 22/deg 2 VLSS g 74 MHz δ > VLA BnA/B 0.1/0.5 Jy 3/deg 2 a planned LOFAR survey at 120 MHz [20] b planned deep LOFAR survey at 200 MHz [20] c Faint Images of the Radio Sky at Twenty-cm [1], area coincident with SDSS d NRAO VLA Sky Survey [7] e Westerbork Northern Sky Survey [19], Westerbork In The Southern Hemisphere f Westerbork Synthesis Radiotelescope g VLA Low-frequency Sky Survey, former name 4MASS, 50 % completed Source-targeted search The planned LOFAR-200 survey (see Tab. 6.1) will find sources, among which there will be lenses (estimated lensing rate ca. 1:2 000 [13]). The challenge is not to find the lenses but to reject the non-lenses reliably to define a candidate sample that can be followed-up with the EVLA and e- MERLIN. Any pre-selection can only be reliable at a S/N > 35 [13], which means that LOFAR-200 sources (with lenses) form the primary source 45

46 sample. Experience with CLASS and new simulations show that a false-positive rate of < 1 % is a realistic goal for image separations of 1.2 times the resolution of the survey [13, 14]. With baselines of 400 km, this corresponds to image separations 1 (Tab. 6.1), which comprises about 68 % (1 000) of the lenses (Figure 6.1). With shorter baselines, the number of detectable lenses declines rapidly, making a representative census of normal lensing galaxies impossible. Only the large-separation tail of the lens distribution could be probed with a short-baseline LOFAR, which would limit its value for studies of galaxy structure and evolution severely. A very-long-baseline-lofar would be needed to use a much larger LOFAR- 120 survey for the source sample. The expected total number of lenses down to S/N 35 is (!), but the identification algorithm has to be even more reliable ( 0.1 % false positives). With a baseline length of km such a project will become feasible. Figure 6.1: Statistics of image separations in CLASS [5]. Lens-targeted search Complementing the blind source-targeted search, it is also possible to search among background sources very close to potential lens galaxies. This strategy has been proven extremely successful in the optical. The SLACS [3] found 19 new lenses with a success rate of 68 % by targeting background sources in the vicinity of luminous red galaxies (LRG) from the SDSS survey [8]. A similar strategy can be followed with LOFAR by combining the LOFAR-120 survey with the LRG sample from SDSS. From the number densities and typical lensing cross-sections, it can be estimated that lenses among candidates can be found in this way. In order to avoid misinterpreting radio emission from the LRG themselves as background radio sources, it is necessary to resolve the galaxies at least marginally. According to Tab. 6.1, this requires baselines 400 km. 6.3 Cluster lensing In addition to galaxy lenses, this lens-targeted strategy can also be used for clusters of galaxies. A rich cluster has a typical lensing cross-section of about 46

47 1 arcmin 2, so that each cluster will produce multiple images of ca. 35 LOFAR-200 sources. In addition, these images are usually highly magnified, total magnifications of 50 should be quite common. Recently, these high magnifications have permitted a few cases of cluster multiple imaging in the radio to be detected using existing instruments. The first case was discovered in Abell 2218 [11] see Figure 6.2. In this case, the brightest image (with a measured redshift of 2.5) is magnified by a factor of 14, implying an intrinsic flux density of only a few microjy. Cluster lensing thus permits the detection of extremely faint (low-luminosity) galaxies that would otherwise only be detectable after many months of observing! Figure 6.2: Multiply imaged, high-redshift (z 2.5) radio sources in A2218 (white circles). Without the magnification boost provided by cluster lensing, these intrinsically faint sources would only be detected after many months of observations using current instruments. The white rectangular also marks the position of a highly magnified but singly imaged background source (z 1) [11]. Figure 6.3: Left: A sub-mm SCUBA map (red contours) superimposed upon an HST image of the rich cluster MS The sub-mm emission is related to multiple images of background sources located at z 2.9, [4]). Right: as left but with a VLA radio map (white contours) also overlayed [2]. The radio and sub-mm emission extend across 1 arcminute the scale and similarity of the emission is quite striking. Rare source-cluster alignments such as these, may provide us with a glimpse of the properties of these intrinsically faint sources (e.g. their Spectral Energy 47