1. Diffuse polarizationimaging (and the case forAAsformid-frequencies !) Ger de Bruyn ASTRON, Dwingeloo& Kapteyn Institute, Groningen 09-December-2010 1 AAVP-Meeting, Cambridge, UK

2. 09-December-2010AAVP-Meeting, Cambridge, UK2 Outline - (Linear) Polarization – science - Some recent resultson diffuse polarization (150-1400 MHz) - Optimalfrequency range: 300 – 1400 MHz using RM synthesis - RelevanceforAA’s, and (what) do we need to prove (it) ? - Conclusions

3. 09-December-2010AAVP-Meeting, Cambridge, UK3 Polarization = Magnetismscience CosmicMagnetism: one of SKA sciencedrivers (Gaensler et al, 2004) (butunderpresssurefor SKA 1 !? ) Magnetism is alsoone of 6 KeyScienceProjectsfor LOFAR (seewww.mpifr-bonn.mpg.de/staff/rbeck/MKSP/mksp.html)www.mpifr-bonn.mpg.de/staff/rbeck/MKSP/mksp.html Somescience goals: -all-sky RM grid (> 10 7 sources) -magneticfields in normalgalaxies - cluster and cosmic-webmagneticfields -magneticfields in pulsars, stars - AGN, giant radio galaxies - ourGalaxy (foregroundfor CMB and EoR !) Taylor, Stil &Sunstrum, 2009 37,453 sources

4. 09-December-2010AAVP-Meeting, Cambridge, UK4 Someresultsondiffusepolarization 22 GHzAll-sky (WMAP) 1400 MHz All-skyvery low resolution (0.5-1 o ) 1400 MHz selectedregions at 0.5-1’ 350 MHz large-sky 4’ resolution 150 MHz FAN region (l=137 o, b=+8 o ) Bottomline: Veryrichpolarizationscience in diffuse (mostlyGalactic) structures. Complex Faraday spectra (Faraday-thin and Faraday-thickcomponents) These need to beunderstoodif we want to properlyinterpret RM grid of discrete sources !

5. Full-sky polarization (high freq) images of our Galaxy 22.8 GHz WMAP image (no depolarization ) 1.4 GHz Reich, Wolleben et al (depolarization effects already visible in the Galactic plane) 09-December-20105AAVP-Meeting, Cambridge, UK

6. 09-December-2010AAVP-Meeting, Cambridge, UK6 Arcminpolarizationstructure in inner Galaxy Gaensler et al (2001) ATCA, 1400 MHz Veryrichpolarizationstructu re in inner Galaxy No orverylittlecorrelationbetw een Q,U and I

7. WSRT 1400 MHz observations Perseus cluster East Central North West 21cm pointings (1300-1460 MHz) 92cm background de Bruyn & Brentjens, 2011)

8. Perseus - EAST Peak in I ~ 5 Jy (3C84) Pol-noise ~ 5-7  Jy/PSF  DR ~ 10 6 : 1 (Relative to an off-axis source !!) All polarized emission probably from Galactic foreground Note ‘embedded’ discrete sources ! 30”x45” smoothedpolarised intensity

9. WSRT 325 MHz polarization in our Galaxy’s 2 d quadrant Schnitzeler et al (2007) Haverkorn et al (2004) 09-December-20109AAVP-Meeting, Cambridge, UK

10. Galactic location of FAN and Abell 2255 Haslam et al (1981) 408 MHz All-sky (0.85 o PSF) Galactic coordinates Equatorial coordinates FAN Abell 2255 09-December-201010AAVP-Meeting, Cambridge, UK

11. Polarization at 150 MHz (!) in FAN region: ‘the ring’ RM = - 5 rad/m 2 RM = - 2 rad/m 2 09-December-201011AAVP-Meeting, Cambridge, UK Bernardi et al, 2009

12. Stokes Q and U images at RM = - 2 rad/m 2 09-December-201012AAVP-Meeting, Cambridge, UK

13. 09-December-2010AAVP-Meeting, Cambridge, UK13 WSRT 350 MHz Cluster of galaxiesAbell 2255 Pizzo et al, 2010

14. 09-December-201014AAVP-Meeting, Cambridge, UK Linearpolarizationtowards A2255: a different picture ! Integratedpolarizedintensity at variousFaradaydepths: -4 to +12 rad/m 2 +16 to +36 rad/m 2 de Bruyn &Pizzo, 2010

15. 09-December-201015AAVP-Meeting, Cambridge, UK Sum of Faradaydepth frames +16 to +36 rad/m 2 PolarisedIntensityStokes Q (U lookssimilar) Note the veryrichangularstructure in pol angle !

16. 09-December-201016AAVP-Meeting, Cambridge, UK Double-valuedFaraday spectra Diffuse emission Φ = +15 and + 42 rad/m 2 (noise bias ~ 60 μJy/beam ) Discrete source: Φ = 0 rad/m2  instrumental Φ = +42 rad/m2  intrinsic

17. 09-December-2010AAVP-Meeting, Cambridge, UK17 1) The wholesky is filledwith diffuse polarization (Galaxy is all aroundus !)  needlargesurvey speed ! 2) Goodresolutionneeded in RM space ( = Faradaydepth)  widefrequency range down to ~ 300 MHz and possibly up to 1400 MHz Thiswillgive excellent resolution in RM (orFaradaydepth) spacethrough RM synthesis (Brentjens & de Bruyn, 2005) RMSF ~ 3.5 /Δ(λ 2) e.g. 300 - 1000 MHz  4 rad/m 2 3) A compact array (< 500m ) couldalready prove the science case AND demonstratetechnical performance (cal, imaging, DR, wide FOV, polar) Polarimetry and the case forAA’s

18. 09-December-2010AAVP-Meeting, Cambridge, UK18 Conclusions - Studyingcosmicmagnetismrequiresunderstandingforegroundemission - Optimalfrequency range: ~ 300 - 1000 MHz - Minimum resolutionrequiredabout 5’  300 - 500m arraycan do it ! - Fastsurvey speed needed  ApertureArray - We need to demonstratethisby 2015 (preaching to the converted?) - (Seriousdoubtsthat a 70 - 450 MHz AA-arraywouldbeinterestingor even possible)