Slide 1

The low frequency Galactic polarisation foreground Xiaohui Sun & Wolfang Reich MPIfR 23.05.2011 Slide 2 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 Slide 3 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 Slide 4 learn from observations: RM synthesis Brentjens & de Bruyn 2005 from Heald Slide 5 data: Bernardi et al. (2009) RM synthesis software: Brentjens clean software: Heald : 9 2 rad m -2 resolution: 3.44 rad m -2 max extent: 0.85 rad m -2 9 5 4 321 012 87 6 Slide 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) 2 rad m -2 Slide 7 9 54 32 87 6 A ring structure! Slide 8 An emission feature? NO! -- the intrinsic ~0 -- only appear at certain 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 Slide 9 HAMMURABI code (Waelkens et al. 2009) pixel Understanding observations by simulations 3D-emssion models Sun 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 Slide 10 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 Slide 11 FS facts D: 200-250 pc R: 6.4 pc Shell: 2 pc : 1.5--3.7 rad m -2 -- RM synthesis ~ 1/ 2 min 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) Slide 12 Previous observations Haverkorn et al. 2003 Schnitzeler et al. 2007 Difficulties observations: influence of large-scale emission simulations: add local emission Slide 13 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 Slide 14 Thank you!