A Sino-German λ6 cm polarization survey of the Galactic plane I. Survey strategy and results for the first survey region

Aims. Polarization measurements of the Galactic plane at λ6 cm probe the interstellar medium (ISM) to larger distances compared to measurements at longer wavelengths, hence enable us to investigate properties of the Galactic magnetic fields and electron density. Methods. We are conducting a new λ6 cm continuum and polarization survey of the Galactic plane covering 10◦ ≤ l ≤ 230◦ and |b| ≤ 5◦. Missing large-scale structures in the U and Q maps are restored based on extrapolated polarization K-band maps from the WMAP satellite. The λ6 cm data are analyzed together with maps at other bands. Results. We discuss some results for the first survey region, 7◦ × 10◦ in size, centered at (l, b) = (125. ◦5, 0◦). Two new passive Faraday screens, G125.6−1.8 and G124.9+0.1, were detected. They cause significant rotation of background polarization angles but little depolarization. G124.9+0.1 was identified as a new faint HII region at a distance of 2.8 kpc. G125.6−1.8, with a size of about 46 pc, has neither correspondence in enhanced Hα emission nor a counterpart in total intensity. A model combining foreground and background polarization modulated by the Faraday screen was developed. Using this model, we estimated the strength of the ordered magnetic field along the line of sight to be 3.9 μG for G124.9+0.1, and exceeding 6.4 μG for G125.6−1.8. We obtained an estimate of 2.5 and 6.3 mK kpc−1 for the average polarized and total synchrotron emissivity towards G124.9+0.1. The synchrotron emission beyond the Perseus arm is quite weak. A spectral curvature previously reported for SNR G126.2+1.6 is ruled out by our new data, which prove a straight spectrum. Conclusions. The new λ6 cm survey will play an important role in improving the understanding of the properties of the magnetoionic ISM. The magnetic fields in HII regions can be measured. Faraday screens with very low electron densities but large rotation measures were detected indicating strong and regular magnetic fields in the ISM. Information about the local synchrotron emissivity can be obtained.