1. T.G.Arshakian MPI für Radioastronomie (Bonn) Exploring the weak magnetic fields with LOFAR

2. Outline Advantages of low frequency radio astronomy Observations of regular magnetic fields Faraday rotation is a powerful tool to detect weak magnetic fields What can LOFAR observe?

3. Advantages of low-frequency (LF) radio astronomy LF emission is purely nonthermal in nearby galaxies, IGM and ISM Radio synchrotron emission is a measure of the strength of the total magnetic field (B tot ): Allows detailed studies for few dozens of nearby galaxies …. Linear polarization – degree of ordering of the magnetic field: Fully ordered field can polarize the signal up to 75%. Small Rotation Measures ( RM ~  n e B || dr) can be measured (  RM ~ -2 ) → weak magnetic fields. RM Synthesis (Brentjens & de Bruyn 2005) – separate RM components from regions along the LOS (from multichannel spectro-polarimetry).

4. Observing weak magnetic fields Synchrotron intensity: I ~ B  1+   -  → 10x smaller B gives same intensity when observing at 100x smaller frequency  (for  ≈1) Synchrotron lifetime:   =50 MHz, B  =10μG: E e =0.6 GeV → t syn ≈ 1.5 10 8 yr   =50 MHz, B  = 3μG: E e =1.8 GeV → t syn ≈ 9 10 8 yr Inverse Compton loss from CMB dominates for weaker fields (<3μG for nearby objects) → Observing at low frequencies traces old, low-energy cosmic-ray electrons in weak magnetic fields → CR electrons may travel to large distances in weak magnetic fields

5. M 51 VLA+Eff 6cm total intensity + B-vectors (Fletcher & Beck) Regular magnetic fields in the disk Weak magnetic fields may exist in the outer disk regions.

6. NGC5775 6cm total+ polarizedintensity (Tüllmann et al. 2000) X-shaped halo field: halo field:Vertical field components increasing with increasing height Regular fields in the halo

7. NGC253 6cm polarized intensity (PhD Heesen 2007) VLA + Effelsberg Disk + halo field X-shaped halo field halo field NGC253 6cm total+ polarizedintensity (PhD Heesen2007) Weak magnetic fields may exist in the outer halo regions.

8. NGC253 6cm polarized intensity (PhD Heesen 2007) VLA + Effelsberg Disk + halo field X-shaped halo field halo field NGC253 6cm total+ polarizedintensity (PhD Heesen2007) A presence of regular magnetic fields in the disks and halos makes Faraday rotation a perfect tool to study the weak magnetic field structure in spiral galaxies

9. Radio halos and their rotation measures are best observed at low frequencies at low frequencies (LOFAR)

10. Faraday rotation Δψ  λ 2 RM

11. Components of Faraday rotation RM = RM IGM + RM cl + RM gal + RM MW + RM ion <1 ≤10000 ≤1000 ≤1000 ≤10 <1 ≤10000 ≤1000 ≤1000 ≤10 rad m -2 rad m -2

12. LOFAR RM Survey (120-240 MHz) LOFAR can measure very low Faraday rotation measures (below ~1 rad m -2 ) and hence very weak magnetic fields:  Face on galaxies (outer disk): RM<10 rad m -2  Galaxy halos, cluster halos, relics, intergalactic filaments n e =10 -3 cm -3, B  =1 μG, L=1 kpc: RM~1 rad m -2 n e =10 -2 cm -3, B  =1 μG, L=100 pc: RM~1 rad m -2  Intergalactic magnetic fields n e =10 -3 cm -3, B  =0.1 μG, L=1 kpc: RM~0.1 rad m -2

13. RM mapping of nearby galaxies: LMC and SMC ~200 RMs behind LMC Mao et al. 2008 Gaensler et al. 2005 Few RMs behind SMC

14. RM mapping of galaxies & clusters: M 31 and Abell 514 RMs of 21 polarized at ~1.4GHz sources shining through M31 (Han et al. 1998) 5 RMs through Abell 514 (Govoni et al. 2001) RMs through 30 clusters (Johnston-Hollitt 2003)

15. RM mapping of the foreground galaxy M31 at 1.4 GHz SKA RM survey (simulation by Bryan Gaensler) RM mapping of nearby galaxies with the SKA SKA RM survey will detect many polarized sources behind nearby galaxies thus allowing the RM mapping of the foreground galaxy and reconstruction of its magnetic field structures.

16. Number counts of polarized background sources at 1.4 GHz With a SKA sensitivity of 0.05 µJy (T~100 h) ~50000 polarized sources behind M 31, tens to hundreds sources behind a galaxy at a distance from 10 Mpc to 100 Mpc (z < 0.025). Number counts per 1 deg 2 (dotted line; Taylor et al. 2007): 1. observed number counts (P > 0.5 mJy) 2. extrapolated to 0.01 mJy (P < 0.5 mJy) With a sensitivity of 0.01 mJy (T~1m) about 1000 polarized sources will be detected towards nearest spiral galaxy M 31. 1 Mpc (M31) 10 Mpc 100 Mpc Stepanov et al. (2008)

17. RM patterns and perspectives for the SKA SKA perspectives ~600 spiral galaxies (<10 Mpc,  p ~0.2 µJy) can be recognized within T ~ 15 min SKA observation time at 1.4 GHz ~60.000 galaxies (  100 Mpc,  p ~0.015 µJy) with T ~ 100 h. ASS+QSS BSS QSS RM reg patterns: p = 20 deg, i = 45 deg ASS RM patterns of galaxies with different magnetic field configurations Recognition of simple structures of regular magnetic fields can be reliably performed from a limited sample of < 50 RM measurements (Stepanov et al. 2008)

18. RM mapping with LOFAR LOFAR (LWA, ASKAP and SKA-AA) can detect smaller RM values and recognize weak galactic and intergalactic magnetic fields if background sources are still polarized at low frequencies (<300 MHz) ???

19. Number counts of pol. sources at 350 MHz Strong depolarization at 350 MHz and lower sensitivity of LOFAR → low number density of polarized sources at 350 MHz Array Source/deg 2 DFA-1h 1 (18+18) IFA-1h 4 (18+18+14) IFA-10h 10 (18+18+14) IFA-100h 25 (18+18+14) 350 MHz data from Haverkorn 2003, Schnitzeler 2008

20. Coma cluster and around Arecibo + DRAO 408 MHz (73cm) (Kronberg et al. 2007) Coma cluster and filaments Diffuse emission: filament (?) Area~50 deg 2 ; 10h ; N BG ~500 Area~3 deg 2 ; 10h ; N BG ~30

21. RM mapping with LOFAR IFA-10h (18+18+14) gives ~10 pol. sources per sq. degree at HB  Is possible only for nearby sources with large angular sizes covering the sky area of several sq. degrees  LOFAR HB frequencies are preferable (DP is lower)  Recognition of magnetic field structures is possible with N BG >20 polarized sources (Stepanov et al 2008) Galaxies: M 31 ( ~3 deg 2 ; N BG ~30) Clusters: (~50 deg 2 ; N BG ~500) – Coma cluster Filaments(?): (~3 deg 2 ; N BG ~30 ) - in the Coma cluster

22. Summary  RM measurements (120 MHz and 240) MHz provide a powerful tool to measure weak magnetic fields.  Recognition and detection of weak magnetic field structures is possible for nearby objects with large angular sizes; distant objects (or small angular sizes) require a lot of observational time.  For objects with small angular sizes diffuse polarized emission has to be detected to measure RM.