Molecular Clouds in External Galaxies and the Efficiency of Star Formation

Abstract

Observations of the molecular cloud distributions in spiral galaxies are reviewed. For the luminous, relatively face-on Sc galaxies, the azimuthally averaged CO distributions are centrally peaked, with H2surface densities which decrease as a function of radius. For the Sb and Sa galaxies, the CO distributions exhibit central CO holes up to 5 kpc across in a significant fraction of the galaxies studied. In galaxies with this CO morphology, the central hole is coincident with the nuclear bulge of the galaxy. Additionally, the radial distributions of CO and13CO emission are similar in 10 Sb and Sc galaxies.