ALMA reveals diverse dust-to-gas mass ratios and quenching modes in old quiescent galaxies

Abstract

Recent discoveries of dust and molecular gas in quiescent galaxies (QGs) up to $z\sim 3$ challenge the long-standing view that the interstellar medium depletes rapidly once star formation ceases, raising key questions of whether dust and gas co-evolve in QGs, and how their depletion links to stellar aging. We present deep Atacama Large Millimeter/submillimeter Array (ALMA) Band~6 continuum and CO(3--2) observations of 17 QGs at $z\sim 0.4$ in the COSMOS field. Using the dust-to-molecular gas mass ratio (${\delta }_{\mathrm{DGR}}$) as a key diagnostic, we trace post-quenching evolution of the cold interstellar medium. Our study triples the number of QGs with direct ${\delta }_{\mathrm{DGR}}$ estimates, constraining 12 systems with stellar population ages of $\sim$5--10 Gyr. For the first time, we show that ${\delta }_{\mathrm{DGR}}$ in QGs ranges from $\sim 8\times$ below to $\sim 2.5\times$ above the canonical value of ${\delta }_{\mathrm{DGR}}\sim 1/100$. Despite uniformly low molecular gas fractions (median $f_{{\mathrm{H}}_2}=M_{{\mathrm{H}}_2}/M_{\star }\sim 4.1\%$), QGs follow diverse evolutionary paths: about half exhibit rapid ($\sim 700$ Myr) exponential dust decline with age, while the rest show mild decline over $\gtrsim$2 Gyr, maintaining elevated ${\delta }_{\mathrm{DGR}}\gtrsim 1/100$. Our results support simulations predictions of dust and molecular gas evolving independently post-quenching, without a preferred quenching mode. This challenges the use of dust continuum as a ${\mathrm{H}}_2$ tracer, implying that quenching cannot be robustly linked to interstellar medium conditions when relying solely on dust or gas.

7 pages main text, 6 figures, 1 table, 4 pages Appendix, submitted to ApJL