The Resolved Properties of Extragalactic Giant Molecular Clouds

TL;DR

This study systematically analyzes the properties of extragalactic giant molecular clouds using high-resolution CO observations, revealing their consistent characteristics across diverse environments and examining their relation to metallicity and star formation theories.

Contribution

It provides a uniform analysis of GMC properties across various galaxies, comparing them to Galactic GMCs and testing theoretical models of star formation and molecular mass estimates.

Findings

GMC properties are consistent across different galaxy types.

Virial CO-to-H2 conversion factor is similar to the Galactic value.

Discrepancies in molecular mass estimates suggest large H2 envelopes.

Abstract

We use high spatial resolution observations of CO to systematically measure the resolved size-line width, luminosity-line width, luminosity-size, and the mass-luminosity relations of Giant Molecular Clouds (GMCs) in a variety of extragalactic systems. Although the data are heterogeneous we analyze them in a consistent manner to remove the biases introduced by limited sensitivity and resolution, thus obtaining reliable sizes, velocity dispersions, and luminosities. We compare the results obtained in dwarf galaxies with those from the Local Group spiral galaxies. We find that extragalactic GMC properties measured across a wide range of environments are very much compatible with those in the Galaxy. We use these results to investigate metallicity trends in the cloud average column density and virial CO-to-H2 factor. We find that these measurements do not accord with simple predictions from photoionization-regulated star formation theory, although this could be due to the fact that we do not sample small enough spatial scales or the full gravitational potential of the molecular cloud. We also find that the virial CO-to-H2 conversion factor in CO-bright GMCs is very similar to Galactic, and that the excursions do not show a measurable metallicity trend. We contrast these results with estimates of molecular mass based on far-infrared measurements obtained for the Small Magellanic Cloud, which systematically yield larger masses, and interpret this discrepancy as arising from large H2 envelopes that surround the CO-bright cores. We conclude that GMCs identified on the basis of their CO emission are a unique class of object that exhibit a remarkably uniform set of properties from galaxy to galaxy (abridged).