Molecules in Galaxies. I. CO Observations in a Spiral Arm of M31

A 4 x 20 arcmin spiral arm region of M31 is mapped in C-12O (J = 1-0), using the Bell Laboratories 7 m antenna. The emission is confined to an arm 1-2 kpc wide. The surface density of H2, averaged over the mapped region, is 2.2 solar masses/sq pc, projected onto the plane of the galaxy. On the outer edge of the arm, the CO shows a systematic velocity deviation of up to 30 km/s. The molecular gas is accelerated as it crosses the spiral arm. The CO spectra show both broad and narrow emission lines, with widths ranging from 8 to 44 km/s FWHM. At about one-fifth of the observed points, the spectra show double emission lines, with peak-to-peak separations of 12-47 km/s. The tendency for the emission lines to be double-peaked may be the result of rapid acceleration of the gas in cloud-cloud collisions.