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！