Transcript Galactic polarization in the direction of the cluster

Diffuse polarizationimaging
(and the case forAAsformid-frequencies !)
Ger de Bruyn
ASTRON, Dwingeloo&
Kapteyn Institute, Groningen
09-December-2010
AAVP-Meeting, Cambridge, UK
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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
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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:
Taylor, Stil &Sunstrum, 2009
-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 !)
09-December-2010
AAVP-Meeting, Cambridge, UK
37,453 sources
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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 !
09-December-2010
AAVP-Meeting, Cambridge, UK
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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
AAVP-Meeting, Cambridge, UK
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Arcminpolarizationstructure in inner Galaxy
Gaensler et al (2001)
ATCA, 1400 MHz
Veryrichpolarizationstructu
re in inner Galaxy
No
orverylittlecorrelationbetw
een Q,U and I
09-December-2010
AAVP-Meeting, Cambridge, UK
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WSRT 1400 MHz observations Perseus cluster
21cm pointings
(1300-1460 MHz)
North
East
West
Central
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
AAVP-Meeting, Cambridge, UK
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Galactic location of FAN and Abell 2255
Haslam et al (1981)
408 MHz
All-sky (0.85o PSF)
Galactic coordinates
FAN
Abell 2255
Equatorial coordinates
09-December-2010
AAVP-Meeting, Cambridge, UK
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Polarization at 150 MHz (!) in FAN region: ‘the ring’
RM = - 5 rad/m2
RM = - 2 rad/m2
Bernardi et al, 2009
09-December-2010
AAVP-Meeting, Cambridge, UK
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Stokes Q and U images at RM = - 2 rad/m2
09-December-2010
AAVP-Meeting, Cambridge, UK
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WSRT 350 MHz
Cluster of galaxiesAbell 2255
Pizzo et al, 2010
09-December-2010
AAVP-Meeting, Cambridge, UK
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Linearpolarizationtowards A2255: a different picture !
Integratedpolarizedintensity at variousFaradaydepths:
+16 to +36 rad/m2
de Bruyn &Pizzo, 2010
-4 to +12 rad/m2
09-December-2010
AAVP-Meeting, Cambridge, UK
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Sum of Faradaydepth frames +16 to +36 rad/m2
PolarisedIntensityStokes Q (U lookssimilar)
Note the
veryrichangularstructure in
pol angle !
09-December-2010
AAVP-Meeting, Cambridge, UK
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Double-valuedFaraday spectra
Diffuse emission
Φ = +15 and + 42 rad/m2
(noise bias ~ 60 μJy/beam )
Discrete source:
Φ = 0 rad/m2  instrumental
Φ = +42 rad/m2  intrinsic
09-December-2010
AAVP-Meeting, Cambridge, UK
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Polarimetry and the case forAA’s
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)
09-December-2010
AAVP-Meeting, Cambridge, UK
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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)
09-December-2010
AAVP-Meeting, Cambridge, UK
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