Resolved Properties of Extragalactic Giant Molecular Clouds

TL;DR

This study examines extragalactic giant molecular clouds across various environments, finding their properties largely resemble those in the Milky Way, with notable differences in low-metallicity galaxies like the Small Magellanic Cloud.

Contribution

It provides a comprehensive analysis of GMC properties in extragalactic systems, challenging simple star formation theories and exploring the CO-to-H2 conversion factor across different metallicities.

Findings

GMC properties are similar across diverse galaxies, including dwarfs.

No clear trend in the CO-to-H2 conversion factor with environment.

Departures from Larson relations observed in the Small Magellanic Cloud.

Abstract

Giant molecular clouds (GMCs) are the major reservoirs of molecular gas in galaxies, and the starting point for star formation. As such, their properties play a key role in setting the initial conditions for the formation of stars. We present a comprehensive combined inteferometric/single-dish study of the resolved GMC properties in a number of extragalactic systems, including both normal and dwarf galaxies. We find that the extragalactic GMC properties measured across a wide range of environments, characterized by the Larson relations, are to first order remarkably compatible with those in the Milky Way. Using these data to investigate trends due to galaxy metallicity, we find that: 1) these measurements do not accord with simple expectations from photoionization-regulated star formation theory, 2) there is no trend in the virial CO-to-H2 conversion factor on the spatial scales studied, and 3) there are measurable departures from the Galactic Larson relations in the Small Magellanic Cloud -- the object with the lowest metallicity in the sample -- where GMCs have velocity dispersions that are too small for their sizes. I will discuss the stability of these clouds in the light of our recent far-infrared analysis of this galaxy, and I will contrast the results of the virial and far-infrared studies on the issue of the CO-to-H2 conversion factor and what they tell us about the structure of molecular clouds in primitive galaxies.