AAS 205th Meeting, 9-13 January 2005
Session 74 ISM II
Oral, Tuesday, January 11, 2005, 10:00-11:30am, California

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[74.04] Giant Molecular Clouds in the Nearby Universe

E. Rosolowsky (U.C. Berkeley)

I present new observations and analysis methods which provide a fresh perspective on extragalactic GMCs. Using recent surveys of GMCs in M33 and M31 in conjunction with other studies of galaxies in the Local Group, I will show how cloud properties vary in different galactic environments. In general, GMCs share more similarities than differences across the Local Group. However, there are significant variations in the mass distributions of these clouds, suggestive of systematic differences in the GMC formation mechanism in different galaxies. GMCs are invariably found on filaments of dense atomic hydrogen, but such filaments do not necessarily imply the presence of GMCs. I will discuss the midplane hydrostatic pressure as a potential ``second parameter'' for GMC formation in disk galaxies and compare it to the role of disk instability. The value of fmol\equiv \Sigma\mathrm{H2}/\Sigma\mathrm{HI} correlates tightly with midplane hydrostatic pressure across a sample of 8 disk galaxies, including M33 and the Milky Way. In an expanded sample of 28 galaxies, fmol=1 at a constant value of the stellar surface density, suggesting the correlation holds for more systems. I will also present high resolution observations of GMCs in the molecule rich galaxy M64. The highest mass GMCs in M64 (M>107~M\odot) have properties significantly different than expected from extrapolations from lower mass clouds. These clouds have higher internal densities and pressures than lower mass clouds. Such high mass clouds may be responsible for the formation of massive star clusters in starburst systems and their substantially different properties may permit the super star cluster formation that is not currently observed in the Milky Way. This work has been supported by NSF grant 0228963 to the Radio Astronomy Laboratory at UC Berkeley and a NASA Graduate Student Research Program Fellowship.

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