Annu. Rev. Astron. Astrophys. 1991. 29: 581-625 Copyright © 1991 by Annual Reviews. All rights reserved

5. LOCAL GROUP GALAXIES

M31 and M33 are prime galaxies in which to study the detailed distribution of molecular clouds, due to their proximity (~ 700 kpc). Because they cover such large areas on the sky, most studies have concentrated on selected regions, either to determine the CO radial distributions, to correlate CO emission with the spiral arms, or to investigate individual GMCs.

The first investigation of the CO in M31 consisted of observations at 90" resolution along the minor axis (Stark 1979), revealing an absence of molecular clouds in the central region and an annulus of emission coincident with the optical arms. Along the minor axis, the inferred H₂ surface density is less than or equal to that of H I at all radii (Brinks 1985). Numerous investigations of the concentration of molecular clouds in spiral arms have been conducted for M31 (Combes et al 1977a, b, Boulanger et al 1984, Ryden & Stark 1986, Lada et al 1988). Ryden & Stark (1986) found an arm-interarm contrast of 7 ± 4 and kinematic evidence of streaming across the spiral arms.

In a recent CO study, in which individual GMCs in M31 were partially resolved, Lada et al (1988) inferred sizes, densities, and masses similar to those of GMCs in the solar neighborhood. Comparison with H I observations led them to conclude that the GMC-H I complexes have total gas (H₂ + H I) masses on the order of 10⁶ M, of which ~ 30% is atomic. They infer that the gas column density threshold for H₂ formation is similar to that estimated for clouds in the Milky Way.

CO emission in the nuclear region of M33 has been observed with both single dish and aperture synthesis out to 3.5' (1 kpc) radius by Wilson & Scoville (1989, 1990). The single dish map shows emission complexes with sizes 200-400 pc and masses 3-10 x 10⁶ M some of which are associated with the two spiral arms in this area of the galaxy. The structures are neither tidally nor virially bound and must therefore be transient associations of smaller clouds. The interferometric measurements do reveal individual GMC-type clouds with masses and CO luminosities consistent with their being self-gravitating (see Section 2.2.2).

The Large and Small Magellanic Clouds are sufficiently nearby that they provide the opportunity of achieving high spatial resolution (1' = 23 pc) relative to the sizes of the molecular clouds within them. The most comprehensive study is that of Cohen et al (1988), who obtained a fully sampled CO map at 8.8' resolution (200 pc) of the central 6° x 6° in the LMC and detected CO emission in approximately 10% of the region studied. They found that emission is dominated by a molecular cloud complex extending several kpc south of 30 Doradus. High velocity CO emission was also seen in this region, possibly arising from gas accelerated by stellar winds and supernovae. An extensive high resolution CO survey of the LMC is currently in progress with the SEST telescope.