Strong gravitational lensing

Strong gravitational lensing Olaf Wucknitz, JIVE, Dwingeloo, NL Mike Garrett, JIVE, Dwingeloo, NL Neal Jackson, Jodrell Bank, UK Dieter Engels, Hamburger Sternwarte, Germany 1 Introduction 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 followup observations in a feasible range. 1 000 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 > 40 000. 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 studies in unprecedented detail. An extended LOFAR is essential for 1

these observations, in order to resolve out the extended emission associated with the foreground cluster gas. A funding proposal for a small research group exploring the possibilities (including simulations with a realistic station distribution) and preparing and conducting a LOFAR based lensing survey has already been submitted to the German Science Foundation (DFG). This would provide 16 person-years, which will mostly be spent on lensing research with LOFAR. 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 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 1 gives an overview of planned LOFAR 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. 2.1 Source-targeted search The planned LOFAR-200 survey (see Tab. 1) will find 30 10 6 sources, among which there will be 15 000 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 3 10 6 LOFAR-200 sources (with 1 500 lenses) form the primary source 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. 1), which comprises about 68 % (1 000) of the lenses (Fig. 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 45 000 (!), but the identification algorithm has to be even more reliable ( 0.1 % false positives). With a baseline length of 1 000 km such a project will become feasible. 2

Table 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 860 10 6 1. 3 14/43 µjy 42 000/deg 2 LOFAR-200 b 200 MHz 250 deg 2 30 10 6 0. 8 4.7/14 µjy 120 000/deg 2 FIRST c 1.4 GHz galactic caps 811 000 5 VLA B 0.15/1 mjy 9 033 deg 2 90/deg 2 NVSS d 1.4 GHz δ > 40 1.8 10 6 45 VLA D/DnC 0.45/2.5 mjy 53/deg 2 WENSS+WISH e 330 MHz δ > +30 230 000 60 WSRT f 4/18 mjy 26 < δ < 9 22/deg 2 VLSS g 74 MHz δ > 30 90 000 80 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 2.2 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 100 000 LRG sample from SDSS. From the number densities and typical lensing cross-sections, we estimate that 500 1 000 lenses among 4 000 9 000 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. 1, this requires baselines 400 km. 3

3 Cluster lensing Figure 1: Statistics of image separations in CLASS [5]. 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 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 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! 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 Distribution, detailed high resolution morphology, angular size etc) long before they become natural targets for next generation telescopes such as the Square Km Array (SKA). Since the FIR-radio correlation is also known to apply at high-z [10] many of these low-luminosity star forming galaxies will also be detected by ALMA. Figure 3 shows the second case of multiple imaging of radio sources lying behind massive clusters [2]. Note the close correspondence between the sub-mm and radio emission. An extended, long baseline LOFAR will be crucial in order to take advantage of massive cluster lensing. In particular, processes that are instrinsic to the cluster intergalactic medium give rise to very diffuse, large scale synchrotron radio emission with a steep spectral index [9]. Long baselines will be required in order to resolve this extended foreground haze. In addition, sub-arcsecond resolution will be essential in order to study the detailed morphology of these sources. A combination of long baselines and cluster 4

Figure 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 3: Left: A sub-mm SCUBA map (red countours) superimposed upon an HST image of the rich cluster MS0451.6-0305. 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. magnification should permit the structure of even the most distant sources to be studied on sub-kpc scales. 5

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