A high resolution radio continuum survey of Messier 31. II: Observations at 1412 MHz, data reduction, and the 37W catalogue of radio sources

The radio continuum emission of the spiral galaxy M31 has been mapped at a wavelength of 21 cm with the Westerbork Synthesis Radio Telescope, at an angular resolution of 23" x 35", corresponding to a linear resolution of 70 x 115 pc. Five fields have been observed which, when combined, cover the main part of the disk of M31. A catalogue of sources inside the M31 area with a flux density larger than 1 mJy (five times the rms noise) is presented. Of the 249 entries, 107 are new detections. A source count based on this catalogue shows no significant excess of sources which might be physically associated with M31. This result is in agreement with the 610 MHz count (Bystedt et al., 1984). The small excess that was found there disappears when new deep 610 Mllz counts in randomly selected fields are taken into account. A preliminary identification of the 21-cm radio sources supports this conclusion. Several HII regions have radio counterparts but the number identified is too small to give a significant excess. The 21-cm fluxes of the HII regions are relatively weak (< 5 mJy). Five supernova remnants have been detected. The extended continuum emission from M31 is mapped using a combination of WSRT measurements and short spacing information taken from an unpublished 17.4-cm survey with the 100-m Effelsberg telescope. Two distinct regions can be recognized, viz, the nuclear complex, and a ringlike structure at roughly 10 kpc from the nucleus, corresponding to the region where the Population I constituents are most prominent. This ring of emission is resolved into several complexes, which correlate closely with the distribution of the Hα emission.