Co Emission from Molecular Clouds in the Central Region of M31

Abstract

We detected \co-line emission from a complex of dark cloud located at about 250 pc from M 31's center, which is the most conspicuous extinction feature within the central few hund red pc as observed in $B,V,R,I$-band CCD imaging. The darkest cloud has a 30-pc size, and the virial mass is estimated to be $\sim 8.7\time s 10^5\Msun$, which is the most massive molecular cloud in the bulge of M31. The conversion factor from CO intensity to \htwo column mass is found to be by an order of magnitude greater than that for galactic molecular clouds. The hydrogen mass-to-color excess ratio is also found to be anomalously large.