Unusually High Gas-to-Dust Ratios Observed in High-Redshift Quiescent Galaxies

TL;DR

This study uses ALMA observations to reveal that high-redshift quiescent galaxies have unusually high gas-to-dust ratios, challenging the reliability of dust continuum as a tracer for molecular gas in these galaxies.

Contribution

It provides the first direct evidence of high GDR in high-redshift quiescent galaxies, questioning the common assumption of a universal GDR in such systems.

Findings

Dust emission is weaker than expected based on CO luminosity.

GDR in these galaxies ranges from 300 to at least 1200.

Dust continuum is unreliable for estimating molecular gas in high-redshift quiescent galaxies.

Abstract

Tracking the cold molecular gas contents of galaxies is critical to understand the interplay between star formation and galaxy growth across cosmic time. Observations of the long-wavelength dust continuum, a proxy for the cold gas, are widely used in the high-redshift community because of their ease and efficiency. These measurements rely on the assumption of a molecular gas-to-dust mass ratio, typically taken to be GDR ~ 100 in massive, metal-rich systems. We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 870um dust continuum in a sample of five massive quiescent galaxies at z ~ 1 with existing detections of CO(2-1). We find surprisingly weak dust emission, falling a factor of >~0.4-0.8 dex below the typical correlation between CO and continuum luminosity. We interpret this dust deficiency as evidence for unusually high GDR in these galaxies, which we calculate to range from 300 to at least 1200. Our results and other observations from the literature are generally compatible with predictions from the SIMBA cosmological simulation that dust is preferentially destroyed in quiescent galaxies. Ultimately, we conclude that the dust continuum is a highly unreliable tracer of the molecular gas in high-redshift quiescent galaxies. As a consequence we may know much less about the cold gas contents of this population than previously thought.