FACE-ON BARRED SPIRAL STRUCTURE OF DARK CLOUDS IN M31S BULGE

By image processing and color-excess analyses of B, V, R, and I-band C CD images of the central region of M31 taken with the Kiso 105-cm Schm idt telescope, we found a bar of 200-pc length. We also found a more e xtended ''face-on'' spiral feature of dark clouds, which appears to be connected to this bar and is probably an out-of-plane structure. (CO) -C-12(j = 1-0)-line emission was detected from the dark clouds using t he Nobeyama 45-m telescope, and the virial mass of individual clouds w as estimated to be approximately 10(6) M.. The CO intensity-to-virial mass ratio is anomalously small, and thus is the color excess-to-mass ratio, which indicates a low temperature and a small amount dust per u nit gas mass in the cloud, respectively. This implies that the molecul ar clouds in the central few hundred pc of M31 are more ''primeval'', suggesting low star formation and low dust supply. We propose a possib le scenario for the origin of a face-on spiral containing such gas on the basis of a ram-pressure accretion model of stripped gas clouds fro m the companion galaxies.