Unusually High Gas-to-dust Ratios Observed in High-redshift Quiescent Galaxies

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 δGDR ≈ 100 in massive, metal-rich systems. We present Atacama Large Millimeter/submillimeter Array observations of the 870 μm 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 δGDR 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.