Face-on Barred Spiral Structure of Molecular Clouds in M31's Bulge

Abstract

By image processing and color excess analyses of B, V, R, and I-band CCD images of the central region of M31 taken with the Kiso 105-cm Schmidt telescope, we found a bar of 200 pc length. We also found a more extended ``face-on" spiral feature of dark clouds, which appear t o be connected to this bar and is probably an out-of-plane structure. The \co-line emission was detected from the dark clouds using the Nobeyama 45m telescope, and the virial mass of individual clouds was estimated to be about 10E6 solar masses. The CO intensity-to-virial mass ratio is anomalously small, and so is the color excess-to-mass ratio, which indicates a low temperature and a small amount dust per unit gas mass in the cloud, respectively. This implies that the molecular clouds in the central few hundred pc of M31 are more primeval, suggesting a low star-formation and low dust supply. We propose a possible scenario for the origin of the face-on spiral containing such gas on the basis of a ram-pressure accretion model of stripped gas clouds from the companion galaxies. Key words: Dark clouds -- CO line -- Galaxies; individual (M31) -- Interstellar dust -- Nuclei of galaxies.