CO Emission from Massive Molecular Clouds in the Inner Disk of M31

Abstract

In a recent paper we reported on the detection of faint, extended ¹²CO emission from dust clouds in the inner disk of M31. We have now observed the ¹³CO emission line from selected positions in these clouds and confirm that the ¹²CO emission in optically thick. The brightness of the ¹²CO line implies excitation temperatures of only 3 – 4 K. The low temperature of this gas is consistent with the low UV flux and low cosmic ray density in these regions of M31; it is likely that the cosmic rays alone control the heating in the interiors of these clouds. Preliminary models indicate that the cloud densities in the region which dominates the emission of the ¹²CO lines are of order 50 H₂ molecules cm^-3. The surface density of molecular gas in these inner-disk cold clouds is similar to that of the warmer gas found much further out in the star-forming ring of M31.

It is becoming clear that our view of the distribution of molecular clouds in galaxy disks has been significantly affected by observational selection favoring the warmer, brighter gas. It is likely that many disk galaxies contain large quantities of cold molecular gas which has previously escaped detection.

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