The polarized disk in M31 at λ6 cm

We present a radio continuum survey of M 31 at lambda6.2 cm with high sensitivity in total power and polarization, observed with the 100-m Effelsberg dish with an angular resolution of 2.'4. (1) Combination with the VLA + Effelsberg map at lambda20.5 cm in total power yielded a spectral index map at 3' resolution. Both the spectrum of the integrated emission and the spectral index distribution across M 31 indicate a nonthermal spectral index alpha(n) = 1.0 +/- 0.2. We derived maps of thermal and nonthermal emission at lambda6.2 cm. Radial profiles of the various emission components north and south of the minor axis revealed that the stronger total emission in the northern part of M 31 is entirely due to stronger thermal emission, whereas the profiles of nonthermal and polarized emission are nearly identical on either side of the minor axis. This suggests that recent star formation does not lead to a local increase of the number of relativistic electrons and/or magnetic field strength. (2) We discuss several properties of the polarized emission from M 31 and their implications for the magnetic field. At lambda6.2 cm the polarized intensity systematically varies along the bright ring of emission which shows that the regular magnetic field, B-reg, is nearly aligned with the spiral arms forming this ring. The variation of the rotation measures between lambda11.1 cm and lambda6.2 cm, RM(11, 6), across the galaxy confirms this alignment. The nonthermal degree of polarization reaches values >50% near the polarization maxima, implying that the magnetic field in M 31 is exceptionally regular. (3) We derived the distribution of the nonthermal depolarization between lambda11.1 cm and lambda6.2 cm, DPn(11, 6), which is a measure of Faraday depolarization. Gradients in RM(11, 6) may be an important cause of Faraday depolarization in M31. The lack of anticorrelation between the thermal emission, which comes mainly from dense H II regions with small filling factors, and RM(11, 6) and DPn(11, 6) indicates that rotation measures and Faraday depolarization originate in the extended diffuse ionized gas.