The low frequency Galactic polarisation foreground

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The low frequency Galactic polarisation foreground
Xiaohui Sun Wolfang Reich MPIfR23.05.2011
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foreground polarisation causes trouble
Galactic foreground
1 rad m-2 ? 229 deg at 150 MHz
0.15 Leakage of PI to I (dotted) reionisation
(solid)
Jelic et al. 2010
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WSRT observations at 150MHz

• Fan region
• 8 bands
• 139.3 141.5 143.7
• 145.9 148.1 150.3
• 152.5 154.7
• bandwidth
• 2.5 MHz
• Frequency resolution
• 9.8 kHz (tapering)
• Resolution (PI)
• 4.2 arcmin
• 1 mJy/beam 1 K
• rms fluctuation 7 K

Bernardi et al. 2009
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learn from observations RM synthesis
from Heald
Brentjens de Bruyn 2005
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• data
• Bernardi et al. (2009)
• RM synthesis software
• Brentjens
• clean software
• Heald

• f
• -9 2 rad m-2
• resolution
• 3.44 rad m-2
• max f extent
• 0.85 rad m-2

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Missing large-scale components

• 2 4 K at 408 MHz
• (Brouw Spoelstra 1976)
• spectral index of 2.5
• ? 24 49 K at 150 MHz

• Faraday depth very small
• (Spoelstra 1984)
• flt0.8 rad m-2
• otherwise depolarisation
• ? assuming f0

Structures not influenced if fgt2 rad
m-2
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A ring structure!
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Properties of the ring structure --
centre-filled to ring-like -- intensity twice as
the surrounding -- inner radius about 1 degree --
outer radius of about 2 degree
An emission feature? NO! -- the intrinsic
f0 -- only appear at certain f
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HAMMURABI code (Waelkens et al. 2009)
Understanding observations by simulations
3D-emssion models Sun et al. 2008 Sun
Reich 2009 Sun Reich 2010
pixel

• Code modifications
• Cartesian coordinates
• Output RM cubes directly
• Add faraday screens
• Zoom-in of a patch

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Observations imply a Faraday screen

• FD from Simulations
• negative
• monotonously decreasing
• versus distance

• Constrains on FS
• positive
• smaller at centre
• FD
• too large
• too small
• distance
• too far
• too near

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FS facts
D 200-250 pc R 6.4 pc Shell 2 pc F 1.5--3.7
rad m-2
-- RM synthesis 1/?2min maximal FD extent
0.85 rad m-2 needs frequency gt 400 MHz
(Effelsberg 300-800 MHz GMIMS survey) --
Geometry of the FS multi-FSs? (Bernardi et
al. 2009)
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Previous observations
Haverkorn et al. 2003
Schnitzeler et al. 2007
Difficulties observations
influence of large-scale
emission simulations add local emission
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Summary

• Nature of the FS
• related with the star HD 20336
• -- B2V
• -- D 24637 pc (parallax)
• -- wind-blow bubble?
• although disfavored by Haverkorn et al.
  (2003)
• not clear
• -- a magnetic structure (Haverkorn et al.
  2003)

• need 300-800MHz observations (GMIMS)
• RM synthesis simulations can constrain
  distance
• and other parameters!
• nature of FSs remains unclear

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Thank you!??!