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Early radio observations of M31 indicated a radio halo around M31, but with higher resolution this feature was resolved into point-like sources and a Galactic spur emerging from the plane of the Galaxy (Gräve et al., 1981). This spur is obvious also on the deep lambda20 cm polarization map obtained with the Effelsberg telescope (Beck et al., 1998). No excess radio continuum emission is detected around M31.

On the other hand, the filaments visible in Fig. 4b indicate that some cosmic rays and magnetic fields may be leaving the ring. Han et al. (1998) found that the RMs of polarized background sources located at radii up to 25 kpc radius from the center of M31 are similar to the RMs of the emission from the ring (Fig. 3) at the same azimuthal angle. It seems that the regular field and the thermal gas extend much beyond the ring and may form a magneto-ionic thick disk or halo. A larger sample of RMs of polarized background sources is needed to confirm this result.