The Origin and Evolution of Cosmic Magnetism: Perspective from SKA Luigina Feretti IRA - Bologna MCCT-SKADS School, Medicina, 25 9-07
This topic is one of the 5 Key Science Projects of SKA, selected by the Science Working Group Motivations: 1. Can address unanswered questions in fundamental (astro)physics 2. Is science which is unique to the radio band and to the SKA 3. Excites the broader community, & is of interest to funding agencies and from a phase-space perspective, will almost certainly yield new and unanticipated results!
Outline - Importance of the study of cosmic magnetism - Observation of large-scale magnetic fields - Current ideas on the origin of cosmic magnetic fields - Studies with SKA and SKA pathfinders
Cosmic Magnetism Magnetism is one of the Fundamental forces in nature. It is crucial in : cloud collapse / star formation stellar activity / stellar outflows ISM turbulence / gas motions supernova remnants stability of galactic disks acceleration / propagation / confinement of cosmic rays heating in galaxy clusters AGNs / Jets Proplyd in Orion SN 1006 MHD turbulence Merger in gal. cluster
Most bodies in the Universe are magnetized on all scales Earth: 0.5 G Interplanetary Space: 50 µg Sun: 10 G (poles) 1000 G (sunspots) Protostars: 1 mg White dwarfs: 10 6 G Neutron stars: 10 12 G Milky Way: 5 µg (widespread) 1 mg (nucleus) Spiral galaxies: 10 µg (average) 30 µg (massive arms) Starburst galaxies: 50 µg Radio galaxies: µg Clusters of galaxies: 0.1-1 µg Intergalactic space: < 10-2 10-3 µg Large-scale fields Challenge to models
Magnetism and Radio Astronomy Most of what we know about cosmic magnetism derives from radio observations 1 - Synchrotron emission total intensity field strength polarization orientation/degree of ordering 2 - Faraday rotation
1 - Synchrotron emission Total intensity : measures the total field strength Polarization: gives the orientation and the degree of ordering of field
Equipartition magnetic field By writing the synchrotron luminosity as the observed source brightness I 0 at the frequency ν 0, and the source depth d (to be inferred), applying the K-correction, assuming Φ = 1 (same volume in particles and magnetic field), and expressing the parameters in commonly used units: u 12 4/ 7 4α / 7 (12+ 4α )/7 4/ 7 min = 1.23x10 (1 + k) ν 0 (1 + z) I0 d 4/ 7 u min in erg/cm 3 ν 0 in MHz I 0 in mjy/arcsec 2 d in kpc Constant computed for α = 0.7, ν 1 = 10 MHz, ν 2 = 100 GHz H eq 24π = 7 u min 1/ 2 Usually k = 0 or k = 1 assumed for clusters BUT see Brunetti et al 1997, Beck and Krause 2005
The synchrotron radiation from a population of relativistic electrons in a uniform magnetic field is linearly polarized, with the electric vector perpendicular to the magnetic field which has generated the synchrotron emission. In the optically thin case, for isotropic electron distribution, and electron power-law energy spectrum: N(E)dE = N 0 E -δ de Polarization the degree of intrinsic linear polarization is 3δ + 3 P Int = 0. 75 0. 8 3δ + 7
The above value is reduced in the more realistic cases where - the magnetic field is not uniform, since regions where the magnetic field has different orientations give radiation with different polarization angle orientations, which tend to average (or cancel) each other. - there is Faraday rotation effect arising both from instrumental limitations (beamwidth bandwidth) or within the source itself (Sokoloff et al. 1998, 1999 : how fractional pol. is affected by magnetic field configurations)
Synchrotron Emission from the Milky Way (Perseus - Auriga) b=+4 b=-4 l=166 Polarized emission Effelsberg 21cm (Reich et al 2003) l=150
M51 VLA +Effelsberg (Fletcher & Beck 2004)
Clusters of galaxies: being the largest systems in the Universe, they represent an ideal laboratory to test theories for the origin of extragalactic magnetic fields Reviews by Carilli & Taylor 2002,Govoni & Feretti 2004
RADIO: WSRT, 90 cm (Feretti et al.1998) Center COMA Cluster B eq 0.4 µg 500 kpc +
Cluster radio halos A665 Coma A2163
u112 Cluster radio relics 0917+75 A548b
Diapositiva 16 u112 rings around the cluster unibo; 15/07/2004
Abell 2256 I 1.4 & B 0 Projected magnetic field direction Polarization degree: large scale order and generally follow the bright filaments large regions (500 kpc) of fairly uniform magnetic field direction Clarke et al. (2004) Results
Intergalactic Fields: Filament of galaxies ZwCl 2341.1+0000 z 0.3 Size 4 Mpc (Bagchi et al. 2002) 320 MHz VLA
Intergalactic Fields (cont.) Upper limits of intergalactic fields from existing studies: B IGM < 10-9 -8 G (model dependent) GRB 000131 at z = 4.5 (Bloom et al 2001) Radio galaxy at z = 5.2 (van Breugel et al 1999)
2 - Rotation measure gives an indirect measurement of the strength and structure of the field along the line of sight
Faraday Rotation rotation of the plane of polarization of linearly polarized emission as it passes through a magneto-ionic plasma -- due to the different phase velocities of the orthogonal circular modes λ 2 χ 0 χ Kronberg 2002 Rotation Measure
Sources seen through a magnetized screen: n e is the electron density in cm -3 L is the path length in kpc B is the line of sight component of the field in µg Infer B along the line of sight in the crossed medium by combining with info about n e from X-rays
Values derived for B are model dependent - analytical solution only for simplest models of the Faraday screen Otherwise: - numerical techniques (Murgia, Govoni, 2004-2005) - semianalytical approach (Ensslin, Vogt 2004-2005)
Numerical Simulations Power spectrum analysis (Ensslin and Vogt 2003 Murgia et al. 2004) simulate a box with 3D multi-scale fields which have a radial decrease in field strength resolution = 3 kpc, magnetic structures from 6 to 770 kpc find n = 1 2 provide the best fit to the data: most of the magnetic field energy resides in the small scales field strength using this approach are a factor ~ 2 lower than the analytical approach assuming smallest RM scale for coherence length Murgia et al. (2004)
Milky Way All-sky RM map (Johnston-Hollitt et al 2002 RED = POSITIVE RM, BLU = NEGATIVE RM RM approximate range: -300, +300 Pulsar RMs + spiral arm field (Han et al 2002) M 31 RMs of 21 polarized sources (Han et al 1998)
Faraday mapping extended, polarized radio sources can be mapped at several frequencies to produce RM maps cd in a poor cooling-core cluster Cygnus A Image courtesy of NRAO/AUI
A2255 Govoni et al. 2006
Magnetic fields at the µg level are ubiquitous in clusters : - coherence scales of 10-100 kpc - large degree of ordering - structure ORIGIN?
When and how were the first magnetic fields generated?
MAGNETIC FIELD Primordial RECOMBINATION Early stars z 10 Protogalaxies z 5 Galaxies AGN z 0.5 z 0.1
Primordial Fields: (Olinto 1998, Grasso & Rubinstein 2001) Created in the exotic ultra-dense stages of the Big Bang physics poorly known, cannot exclude the creation of a magnetic field of the order 10-30 10-25 G Remember present large scale fields : 10-6 G
Primordial fields would affect the cosmogonic process anisotropic expansion effects on nucleosynthesis (larger He abundance) regulate structure formation
Post-recombination Fields: 1 Early Stars (z 20) 2 First AGN (z 5?) 3 Protogalaxies and structure formation (z 5) (Kulsrud et al 1997, Kang et al. 1997) Seed fields
Seed Fields (Rees 2004) Injection by galactic winds or active galaxies : Kronberg et al.1999, Völk & Atoyan 1999
Present-day fields of B 1 µg G could have evolved from B ~ 10-9 10-10 G seed fields at z > 5 Large-scale fields represent a problem because the dynamo amplification time can be large so not many e-foldings at the present epoch Amplification : dynamo action compression cluster mergers
Square Kilometer Array Very powerful in the detection of total intensity and polarized emission and in RM measurements SKA: instant RMs and position angles: ν = 1.4 GHz, ν = 400 MHz - for t = 1 hour, 1 σ = 0.1 µjy - for P = 1 µjy : RM 5 rad/m -2, χ 10 o!
SKA Faraday Rotation Survey Five min observation with SKA at 1.4 GHz RMs down to P ~ 3 µjy (S tot ~ 0.1 mjy) Approx 500 RMs per deg 2 (average separation ~2-3 ) 10 7 sources over the entire sky, spaced by 90 ( 20000 pulsars) Adapted from Gaensler et al. (2001) & Hopkins et al. (2003)
Scientific breakthrough: - magnetic field of the Galaxy - magnetic field in nearby galaxies and clusters - extended sources
Polarization Silhouettes Distant galaxies are too small to be probed by RM grid but can be probed by Faraday rotation and depolarization of extended background sources NGC 1310 e.g. NGC 1310 against Fornax A (Fomalont( et al 1989) Larger distances: e.g. PKS 1229 021: absorber at z = 0.395 with B ~ 1 1 4 µg (Kronberg et al 1992) Polarization from Fornax A (Fomalont et al 1989) powerful probe of evolution of galactic magnetism as function of redshift Kronberg et al (1992)
Ly-α Absorbers at z ~ 1 3 Large statistical samples can come from RMs and redshifts of quasars (e.g. Welter et al 1984; Oren & Wolfe 1995) - trend of RM vs z probes evolution of B in Ly-α clouds but Galactic contamination, limited statistics Quasar RMs with SKA: - ~10 6 measurements - identification & redshifts from SDSS & successors - accurate foreground removal using RM grid magnetic field evolution in galaxies over cosmic time-scales RRM ~ (1+z) β-2 Residual RMs (Galaxy corrected) vs z of QSOs embedded in intervening clouds (Welter et al 1984) : marginal evidence of evolution!
Magnetic Fields in Protogalaxies thousands of normal spiral galaxies at z ~ 3 detectable with the SKA (1.4 GHz : size = 1-3, flux 0.2µJy ) their radio flux strongly depends on field strength and on star formation rate (and may be polarized) HDF galaxies with z > 4 (Driver et al 1998)
The Magnetized IGM: Cosmic Web Existing limits (scale and model dependent): B IGM < 10-8 -10-9 G (e.g..blasi B IGM e.g..blasi et al 1999; Jedamzik et al 2000) - Detection and polarimetry of very low Level synchrotron emission z = 1 z = 0.5 -RM measurements of extragalactic sources are related to the amplitude and shape of the magnetic field power spectrum P(k) where k is the wave number of the coherence scale z = 2 SKA + z surveys can provide magnetic power spectrum of the Universe RM pairs at separation γ needed to detect B = 1 ng at scale of 50 Mpc (Kolatt 1998)
SKA Specifications for Polarimetry Frequency: at least 1 1010 GHz,, 0.3 20 GHz ideal Large field of view: >1 deg 2 at a resolution of <1" High sensitivity: <0.1 mjy,, confusion limited Large bandwidth: >400 x 1 MHz at 1.4 GHz Significant concentration ( > 50% ) of antennae in central core ( ~ 5 km) High polarization purity ( 40 db at field center, 30 db at field edges)
SKA pathfinders: ATA (US) LOFAR (The Netherlands + Europe) LWA (US) KAT/MeerKAT (South Africa) MWA (Australia) MIRANDA (Australia + Canada) SKADS (Europe)
Low frequency - Diffuse synchrotron emission of steep spectrum - Polarized emission sources of low RM weak magnetic fields
θ = 10 o ν = 240 MHz, ν = 32 MHz RM = 0.4 rad/m 2 λ 2
Conclusions Early primordial fields could have been generated by battery effects, during inflation or phase transitions A primordial intergalactic (IGM) field may have regulated structure formation in the early Universe Seed fields at z > 5 may originate from primordial fields or from post-recombination fields Present-day large-scale fields of B 1 µg could have evolved from B 0 ~ 10-9 10-10 G seed fields at z > 5 Evolution from seed fields includes dynamo, compression, merger interaction
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Biermann Battery effect Electrostatic equilibrium Widrow 2002 When gradients of electron thermodynamic quantities (e.g. density and temperature) are not parallel to the pressure gradient, the electrostatic equilibrium is no longer possible. This leads to a current which generates A magnetic field restoring the force balance. First observed in the lab in 1975 (Stamper & Ripin)
Zeeman effect In a vacuum, the electronic energy levels of an atom are independent of the direction of its angular momentum. In the presence of magnetic fields, the atomic energy levels are split into a larger number of levels and the spectral lines are also split.
u116 Hydrogen Bohr magneton The Zeeman effect can be interpreted as due to the precession of the orbital angular momentum vector in the magnetic field. The energy shift is proportional to the strength of the magnetic field. Zeeman splitting in Hydrogen (1.4 GHz): 2.8 Hz µg -1 Zeeman splitting in the H 2 O molecule (22 GHz): 10-3 Hz µg -1 Lines are polarized, favouring their detection present detection only for strong magnetic fields (> mg) (sunspots + galactic objects)
Diapositiva 51 u116 polarizzazione delle righe splittate e' tale che nel parametro di stokes V sono praticamente opposte quindi si puo' detettare anche se la diff di lunghezza d'onda e' poca unibo; 07/05/2003