Comparative Analysis of Molecular Clouds in M31, M33, and the Milky Way

We present BIMA observations of a 20 field in the northeastern spiral arm of M31. In this region we find six giant molecular clouds (GMCs) that have a mean diameter of 57 13 pc, a mean velocity width of 6:5 1:2 km s , and a mean molecular mass of 3:0 1:6 ð Þ ; 10 M . The peak brightness temperature of these clouds ranges from 1.6 to 4.2 K. We compare these clouds to clouds in M33 observed by Wilson & Scoville using the OVRO millimeter array and some cloud complexes in theMilkyWay observed byDame and coworkers using the CfA 1.2 m telescope. In order to properly compare the single-dish data to the spatially filtered interferometric data, we project several well-known MilkyWay complexes to the distance of Andromeda and simulate their observation with the BIMA interferometer. We compare the simulated Milky Way clouds with the M31 andM33 data using the same cloud identification and analysis technique and find no significant differences in the cloud properties in all three galaxies. Thus, we conclude that previous claims of differences in themolecular cloud properties between these galaxiesmay have been due to differences in the choice of cloud identification techniques. With the upcoming CARMA telescope, individual molecular clouds may be studied in a variety of nearby galaxies. With ALMA, comprehensive GMC studies will be feasible at least as far as the Virgo cluster. With these data, comparative studies of molecular clouds across galactic disks of all types and between different galaxy diskswill be possible. Our results emphasize that interferometric observations combinedwith the use of a consistent cloud identification and analysis technique will be essential for such forthcoming studies that will compare GMCs in the Local Group galaxies to galaxies in the Virgo cluster. Subject headinggs: galaxies: individual (M31, M33, Milky Way) — galaxies: ISM — Local Group — radio lines: ISM Online material: color figure