Published in "Protostars and Planets V", B. Reipurth, D. Jewitt, and K. Keil (eds.), University of Arizona Press, Tucson, 951 pp., 2007., p.81-96.
astro-ph/0602600

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Abstract: We present the first comparative study of extragalactic GMCs using complete data sets for entire galaxies and a uniform set of reduction and analysis techniques. We present results based on CO observations for the LMC, SMC, M33, M31, IC 10 and the nucleus of M64, and make comparisons with archival Milky Way observations. Our sample includes large spirals and dwarf irregulars with metallicities that vary by an order of magnitude. GMCs in H I rich galaxies are seen to be well-correlated with H I filaments that pervade the galactic disks, suggesting that they form from pre-existing H I structures. Virial estimates of the ratio of CO line strength to H₂ column density, X_CO, suggests that a value of 4 × 10²⁰ cm^-2 (K km s^-1)^-1 is a good value to use in most galaxies (except the SMC) if the GMCs are virialized. However, if the clouds are only marginally self-gravitating, as appears to be the case judging from their appearance, half the virial value may be more appropriate. There is no clear trend of X_CO with metallicity. The clouds within a galaxy are shown to have the about the same H₂ surface density and differences between galaxies seem to be no more than a factor of ~ 2. We show that hydrostatic pressure appears to be the main factor in determining what fraction of atomic gas is turned into molecules. In the high-pressure regions often found in galactic centers, the observed properties of GMCs appear to be different from those in the found in the Local Group. From the association of tracers of star formation with GMCs in the LMC, we find that about 1/4 of the GMCs exhibit no evidence of star formation and we estimate that the lifetime of a typical GMC in these galaxies is 20-30 Myr.

Table of Contents

• INTRODUCTION
• THE GALAXIES
• The LMC
• The SMC
• M33
• IC 10
• The Correlation with H I
• Implications for GMC Formation
• MOLECULAR CLOUD PROPERTIES
• THE ROLE OF HYDROSTATIC PRESSURE
• GMCS IN STARBURST GALAXIES
• GMC Formation in Galactic Centers
• STAR FORMATION IN EXTRAGALACTIC GMCs
• Identification of Star Formation
• The Large Magellanic Cloud
• The Evolution of GMCs in the LMC
• Star Formation in M 33
• FUTURE PROSPECTS
• SUMMARY AND CONCLUSIONS
• REFERENCES