Distribution and Motion of CO in M31

Abstract

M31 is among the few galaxies whose solid angle is more than 100 times the beam area of currently operational millimeter-wave telescopes. Roughly 10% of its area has been mapped in the J=1→0 line of CO with the 7-m telescope at Bell Laboratories. The data lie on the major and minor axes and in a filled 20′ by 15′ field. The average CO emissivity is less than 1/5 that of the Milky Way Galaxy, and the inferred H2 column densities are less than 10% of the HI column densities at the same positions. Thus M31 has a predominantly atomic interstellar medium, whereas our Galaxy seems to have a predominantly molecular interstellar medium. The molecular emission is strongly concentrated in spiral-arm segments, with greater than 25:1 contrast between arm and interarm regions. Overall, the CO emission is well correlated with the HI emission, but shows greater concentration to the spiral arms. There is no evidence for a spatial separation of the HI and CO arms to within our resolution (~1 ′): all the spiral-arm tracers are spatially coincident. Almost all the gas on the minor axis exhibits non-circular velocities of about 20 km s−1. There are systematic streaming motions as large as 80 km s−1 associated with spiral arms.