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We present a fully-sampled map of M31 in the CO (J = 2 $\rightarrow$ 1) transition, for a 2' $\times$ 3' section of the southwest arm-interarm region centered at major and minor axis coordinates (-42.0,+9.0), made with the NRAO$^{\ast}$ 12 meter telescope. The map is sampled at intervals of one-half beamwidth, with an angular resolution of 30", or $100 \times 460$ pc along the M31 major and minor axes, respectively. We observed 101 positions, with an rms noise level less than 20 mK (on the ${\rm T_{R}}^{\ast}$ scale) at most positions, and with an effective velocity resolution of $3.9$ km/s.
We identify three distinct regions of CO emission. Along the optical arm, as delineated by HII regions, the spectra are characterized by strong narrow lines; these may be interpreted as giant molecular clouds with masses of a few $\times 10^{5} {\rm M}_{\odot}$. Away from the arm, the spectra are much weaker, indicating an upper limit to the cloud mass in the interarm region of a few $ \times 10^{4} {\rm M}_{\odot}$ . On the inner edge of the optical arm, corresponding to a prominent dust lane, the CO spectra are very strong and have large line widths, and exhibit a pronounced position-to-position similarity of emission, suggesting that these spectra arise from a large number of smaller clouds rather than a few GMCs. As the emission in this region comprises $>$ 50\% of the total CO luminosity in the survey area, we tentatively conclude that most of the molecular material in this region resides in clouds of mass $<$ $ 10^{5} {\rm M}_{\odot}$ , in contrast to the molecular mass distribution in the Milky Way.
The implications for GMC formation and star formation in spiral arms will be discussed.
\noindent D. Mizuno was aided by the travel support program of the Astronomical Society of New York.
\noindent ${\rm {}^{\ast}}$ The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation.
