Diffuse polarizationimaging(and the case forAAsformid-frequencies !)

Ger de Bruyn

ASTRON, Dwingeloo&Kapteyn Institute, Groningen

09-December-2010 1AAVP-Meeting, Cambridge, UK

09-December-2010 AAVP-Meeting, Cambridge, UK 2

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

09-December-2010 AAVP-Meeting, Cambridge, UK 3

Polarization = Magnetismscience

CosmicMagnetism: one of SKA sciencedrivers (Gaensler et al, 2004)

(butunderpresssurefor SKA1 !? )

Magnetism is alsoone of 6 KeyScienceProjectsfor LOFAR

(seewww.mpifr-bonn.mpg.de/staff/rbeck/MKSP/mksp.html)

Somescience goals:

-all-sky RM grid (> 107sources)-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

09-December-2010 AAVP-Meeting, Cambridge, UK 4

Someresultsondiffusepolarization

22 GHzAll-sky (WMAP)

1400 MHz All-skyvery low resolution (0.5-1o)

1400 MHz selectedregions at 0.5-1’

350 MHz large-sky 4’ resolution

150 MHz FAN region (l=137o, b=+8o)

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 !

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-2010 5AAVP-Meeting, Cambridge, UK

09-December-2010 AAVP-Meeting, Cambridge, UK 6

Arcminpolarizationstructure in inner Galaxy

Gaensler et al (2001)

ATCA, 1400 MHz

Veryrichpolarizationstructure in inner Galaxy

No orverylittlecorrelationbetween Q,U and I

WSRT 1400 MHz observations Perseus cluster

East

Central

North

West

21cm pointings (1300-1460 MHz)

92cm background

de Bruyn & Brentjens, 2011)

Perseus - EAST

Peak in I ~ 5 Jy (3C84)

Pol-noise ~ 5-7 Jy/PSF

DR ~ 106 : 1

(Relative to an off-axis source !!)

All polarized emission probably

from Galactic foreground

Note ‘embedded’ discrete sources

!

30”x45” smoothedpolarised intensity

WSRT 325 MHz polarization in our Galaxy’s 2d quadrant

Schnitzeler et al (2007)

Haverkorn et al (2004)

09-December-2010 9AAVP-Meeting, Cambridge, UK

Galactic location of FAN and Abell 2255

Haslam et al (1981)

408 MHz

All-sky (0.85o PSF)

Galactic coordinates

Equatorial coordinates

FAN

Abell 2255

09-December-2010 10AAVP-Meeting, Cambridge, UK

Polarization at 150 MHz (!) in FAN region: ‘the ring’

RM = - 5 rad/m2 RM = - 2 rad/m2

09-December-2010 11AAVP-Meeting, Cambridge, UK

Bernardi et al, 2009

Stokes Q and U images at RM = - 2 rad/m2

09-December-2010 12AAVP-Meeting, Cambridge, UK

09-December-2010 AAVP-Meeting, Cambridge, UK 13

WSRT 350 MHz Cluster of galaxiesAbell 2255

Pizzo et al, 2010

09-December-2010 14AAVP-Meeting, Cambridge, UK

Linearpolarizationtowards A2255: a different picture !

Integratedpolarizedintensity at variousFaradaydepths:

-4 to +12 rad/m2 +16 to +36 rad/m2

de B

ruyn

&Pi

zzo,

201

0

09-December-2010 15AAVP-Meeting, Cambridge, UK

Sum of Faradaydepth frames +16 to +36 rad/m2

PolarisedIntensityStokes Q (U lookssimilar)

Note the veryrichangularstructure in pol angle !

09-December-2010 16AAVP-Meeting, Cambridge, UK

Double-valuedFaraday spectra

Diffuse emissionΦ = +15 and + 42 rad/m2

(noise bias ~ 60 μJy/beam )

Discrete source:

Φ = 0 rad/m2 instrumentalΦ = +42 rad/m2 intrinsic

09-December-2010 AAVP-Meeting, Cambridge, UK 17

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(λ2)e.g. 300 - 1000 MHz 4 rad/m2

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

09-December-2010 AAVP-Meeting, Cambridge, UK 18

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)