Galaxies can be broadly divided into two categories: those actively forming stars and those no longer forming stars. Why do galaxies find themselves in the latter category? Is it because their gas cannot cool and collapse into new stars or because they simply have no more gas? To answer this question we must have robust observations of the gas content of galaxies. In many massive galaxies, the ratio of gas to dust is approximately constant. The thermal emission from dust is also easily observable with radio telescopes such as the Atacama Large Millimeter Array. Together, these two have led to observations of dust emission being a workhorse for estimating the gas masses within galaxies.
Observations of many so-called “quiescent” galaxies (i.e., those not currently forming stars) has shown them to have little dust. The traditional interpretation is that this means they also have little gas, providing a possible explanation for why they can no longer form stars. However, in “High Molecular-Gas to Dust Mass Ratios Predicted in Most Quiescent Galaxies” (Whitaker+) we use a suite of galaxy formation simulations to look at the dust and gas content of simulated quiescent galaxies. We find that the simulated galaxies have extreme gas to dust ratios, such that they are deficient in dust while still having a significant amount of gas present. This is predicted to be caused by the dust being destroyed or depleted much more quickly than the molecular gas in galaxies and has clear observational tests.
The paper has been accepted for publication in the Astrophysical Journal Letters and a preprint is available at the link above.
Update (2021-12-19): The December 2021 issue of “Physics Today” contains a Search & Discovery article, “Ten billion years ago, galaxies were already running out of gas”, discussing the results of this study.