ALMA Reveals Diverse Dust-to-gas Mass Ratios and Quenching Modes in Old Quiescent Galaxies

Recent discoveries of dust and molecular gas in quiescent galaxies (QGs) up to z similar to 4 challenge the long-standing view that the interstellar medium depletes rapidly once star formation ceases, raising key questions of whether dust and gas coevolve in QGs, and how their depletion links to stellar aging. We present deep Atacama Large Millimeter/submillimeter Array Band 6 continuum and CO(3-2) observations of 17 QGs at z similar to 0.4 in the COSMOS field. Using the dust-to-molecular gas mass ratio (delta DGR) as a key diagnostic, we trace postquenching evolution of the cold interstellar medium. Our study triples the number of QGs with direct delta DGR estimates, constraining 12 systems with stellar population ages of similar to 5-10 Gyr. For the first time, we show that delta DGR in QGs ranges from similar to 8x below to similar to 2.5x above the canonical value of delta DGR similar to 1/100. Despite uniformly low molecular gas fractions (median fH2=MH2/M star similar to 4.1% ), QGs follow diverse evolutionary paths: about half exhibit rapid (similar to 700 Myr) exponential dust decline with age, while the rest show mild decline over greater than or similar to 2 Gyr, maintaining elevated delta DGR greater than or similar to 1/100. Our results support simulations' predictions of dust and molecular gas evolving independently postquenching, without a preferred quenching mode. This challenges the use of dust continuum as a H2 tracer, implying that quenching cannot be robustly linked to interstellar medium conditions when relying solely on dust or gas.