Molecular Clouds in Spiral Galaxies

Abstract

We are conducting a large observational program investigating the 2.6 mm CO line in spiral galaxies using the 14 m telescope of the Five College Radio Astronomy Observatory (HPBW = 50″). Thus far we have observed 46 galaxies of types Sa, Sb, Sc and Irr, detected 31, and mapped 16. The major findings from our CO observations are: (1) In several late type spiral galaxies (IC 342, NGC 6946 and M51) the radial distribution of molecular gas out to 10 kpc follows the exponential blue luminosity profile of the disk within each galaxy (Young and Scoville 1982a, Scoville and Young 1982). (2) From a comparison of the CO and blue luminosities of the central 5 kpc in a larger sample of Sc galaxies, we find that the blue luminosity is proportional to the first power of the CO content in both the nuclei and disks of Sc galaxies (Young and Scoville 1982b). We interpret this to mean that the star formation rate per H 2 in Sc galaxies (indicated by the blue luminosity) is constant. (3) No molecular rings like the one in the Milky Way at radii 4 to 8 kpc were seen in the Sc galaxies. (4) We have found molecular rings in two Sb galaxies, NGC 7331 and NGC 2841, with peaks at radii of 4–5 kpc (Young and Scoville 1982c). The central holes in the CO distributions are possibly related to the presence of large nuclear bulges in these galaxies.