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

For a PDF version of the article, click here.

GIANT MOLECULAR CLOUDS IN LOCAL GROUP GALAXIES

Leo Blitz
University of California, Berkeley

Yasuo Fukui
Nagoya University

Akiko Kawamura
Nagoya University

Adam Leroy
University of California, Berkeley

Norikazu Mizuno
Nagoya University

Erik Rosolowsky
Harvard-Smithsonian Center for Astrophysics


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, XCO, 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 XCO 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

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