The low frequency Galactic polarisation foreground

The low frequency Galactic polarisation foreground Xiaohui Sun & Wolfang Reich MPIfR23.05.2011

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

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

learn from observations: RM synthesis from Heald Brentjens & de Bruyn 2005

−9 −8 −7 • data: • Bernardi et al. (2009) • RM synthesis software: • Brentjens • clean software: • Heald −4 −6 −5 • φ: • −9 – 2 rad m-2 • resolution: • 3.44 rad m-2 • max φ extent: • 0.85 rad m-2 −3 −2 −1 0 1 2

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 −8 −7 −6 −5 −4 −3 −2 A ring structure!

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 φ~0 -- only appear at certain φ

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

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 facts D: 200-250 pc R: 6.4 pc Shell: 2 pc Φ: 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 Difficulties observations: 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！

The low frequency Galactic polarisation foreground