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

1. INTRODUCTION

Molecular gas is critical in determining both the morphology and evolution of galactic disks. It is within the giant molecular clouds that interstellar gas is cycled into the next generation of stars, and the most massive of these young stars produce a major part of the galactic luminosity. In addition, the dense interstellar medium, as it is highly dissipative, probably plays a fundamental part in determining the outcome of galactic interactions. Over the last two decades, CO observations have been used to probe the molecular component of hundreds of galaxies from the Local Group to the Virgo cluster, and in luminous galaxies with recession velocities up to cz = 45,000 km s^-1.

The studies of molecules in galaxies include both detailed analyses of nearby galaxies and comparisons of the global properties of selected samples of galaxies. These two approaches are complementary, and both are necessary to improve our understanding of the large-scale processes that govern star formation and molecular cloud evolution. The CO observations are now sufficient to address statistically the global H₂ content of galaxies in relation to other components of the interstellar medium (ISM) and the stellar populations, as a function of morphological type, luminosity, and environment. Specifically:

1. What is the range of H₂ masses in galaxies, and is there a dependence on galaxy type, luminosity, or environment?

2. What is the ratio L / M_gas, or the yield of young stars per unit mass of gas (i.e. the star formation efficiency), within individual galaxies and from galaxy to galaxy?

3. What is the range of M(H₂) / M(H 1) in galaxies. and is there a dependence on galaxy type, luminosity, or environment?

4. What are the effects of spiral arms on the multiphase interstellar gas and the formation of stars?

5. What is the role of the molecular gas component in galactic interactions, starburst galaxies, and active galaxy nuclei?

The answers to these questions depend on multiwavelength analyses of galaxies. CO observations are used to deduce the masses and distributions of molecular hydrogen in galaxies. which are then compared with other tracers (optical and infrared continuum and H and H I line radiation). In Section 2, we first discuss CO as a tracer of H₂ mass in spiral galaxies, including new evidence that global H₂ masses for luminous spirals are generally accurate to ± 50% (1 ), including both measurement uncertainties and variations galaxy to galaxy in the CO to H₂, conversion constant (Devereux & Young 1990b). We then review the radial distributions of gas and star formation (Section 3), spiral structure (Section 4), molecular properties of galaxies in the Local Group (Section 5), global gas contents (Section 6), star formation rates and efficiencies (Section 7), effects of galactic interactions (Section 8), and nuclear gas concentrations (Section 9).