the low frequency galactic polarisation foreground
DESCRIPTION
The low frequency Galactic polarisation foreground. Xiaohui Sun & Wolfang Reich MPIfR 23.05.2011. 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. - PowerPoint PPT PresentationTRANSCRIPT
The low frequency Galactic The low frequency Galactic polarisation foregroundpolarisation foreground
Xiaohui Sun & Wolfang ReichMPIfR
23.05.2011
foreground polarisation causes trouble
Jelic et al. 2010
Galactic foreground
0.15% Leakage of PI to I (dotted) + reionisation (solid)
1 rad m-2 229 deg at 150 MHz
WSRT observations at 150MHz
Bernardi et al. 2009
• 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
learn from observations: RM synthesis
Brentjens & de Bruyn 2005
from Heald
• data: Bernardi et al. (2009)•RM synthesis software: Brentjens• clean software: Heald
• φ: −9 – 2 rad m-2 •resolution: 3.44 rad m-2 • max φ extent: 0.85 rad m-2
−9
−5 −4
−3 −2 −1
0 1 2
−8 −7
−6
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)• φ<0.8 rad m-2
otherwise depolarisation assuming φ=0
Structures not influenced if |φ|>2 rad m-2
−9
−5 −4
−3 −2
−8 −7
−6
A ring structure!
An emission feature? NO!
-- the intrinsic φ~0
-- only appear at certain φ
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
HAMMURABI code (Waelkens et al. 2009)
pixel
Understanding observations by simulations
3D-emssion modelsSun et al. 2008; Sun & Reich 2009; Sun & Reich 2010
Code modifications• Cartesian coordinates• Output RM cubes directly• Add faraday screens• Zoom-in of a patch
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
FS factsD: 200-250 pcR: 6.4 pcShell: 2 pcΦ: 1.5--3.7 rad m-2
-- RM synthesis ~ 1/λ2min
maximal FD extent 0.85 rad m-2 needs frequency > 400 MHz (Effelsberg 300-800 MHz GMIMS survey)-- Geometry of the FS multi-FSs? (Bernardi et al. 2009)
Previous observations
Haverkorn et al. 2003 Schnitzeler et al. 2007
Difficultiesobservations: influence of large-scale emission
simulations: add local emission
Summary
Nature of the FS
•related with the star HD 20336
-- B2V
-- D: 246±37 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
Thank you!
谢谢!