A Sino-German λ6 cm polarization survey of the Galactic plane

III. The region from 10° to 60° longitude

¹ National Astronomical Observatories, CAS, Jia-20 Datun Road, Chaoyang District, Beijing 100012, PR China
e-mail: xhsun@nao.cas.cn; hjl@nao.cas.cn; wangchen@nao.cas.cn
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: xhsun@mpifr-bonn.mpg.de; wreich@mpifr-bonn.mpg.de; preich@mpifr-bonn.mpg.de; rwielebinski@mpifr-bonn.mpg.de; pmueller@mpifr-bonn.mpg.de

Received: 13 July 2010
Accepted: 25 November 2010

Abstract

Aims. We study polarized short-wavelength emission from the inner Galaxy, which is nearly invisible at long wavelengths because of depolarization. We attempt to obtain information on the diffuse continuum emission at short wavelengths to help us to separate Galactic thermal and non-thermal components.

Methods. We conducted a total intensity and polarization survey of the Galactic plane at λ6 cm using the Urumqi 25 m telescope for the Galactic longitude range of 10° ≤ l ≤ 60° and the Galactic latitude range of  |b|  ≤ 5°. Missing absolute zero levels of Stokes U and Q maps were restored by extrapolating the WMAP five-year K-band polarization data. For total intensities, we recovered missing large-scale components by referring to the Effelsberg λ11 cm survey.

Results. Total intensity and polarization maps are presented with an angular resolution of and a sensitivity of 1 mK T_B and 0.5 mK T_B in total and polarized intensity, respectively. The λ6 cm polarized emission in the Galactic plane originates within about 4 kpc distance, which increases for polarized emission out of the plane. The polarization map shows “patches”, “canals”, and “voids” with no correspondence in total intensity. We attribute the patches to turbulent magnetic field cells. Canals are caused by abrupt variation in the polarization angles at the boundaries of patches rather than by foreground Faraday screens. The superimposition of foreground and Faraday screen rotated background emission almost cancels the polarized emission locally, such that polarization voids appear. By modelling the voids, we estimate the Faraday screen’s regular magnetic field along the line-of-sight to be larger than about 8 μG. We separated thermal (free-free) and non-thermal (synchrotron) emission according to their different spectral indices. The spectral index for the synchrotron emission is based on WMAP polarization data. The fraction of thermal emission at λ6 cm is about 60% in the plane.

Conclusions. The Sino-German λ6 cm polarization survey of the inner Galaxy provides new insights into the properties of the magnetized interstellar medium for this very complex Galactic region, which is Faraday thin up to about 4 kpc in the Galactic plane. Within this distance polarized patches are identified as intrinsic structures related to turbulent Galactic magnetic fields of spatial scales from 20(d/4 kpc) to 200(d/4 kpc) pc.