High Molecular-gas to Dust Mass Ratios Predicted in Most Quiescent Galaxies

Katherine E Whitaker,Justin S Spilker,Lamiya Mowla,Christina C Williams,Neal Katz,Romeel Davé,Alexandra Pope,Sune Toft,Rachel Bezanson,Georgios E Magdis,Hollis B Akins,Joel Leja,Desika Narayanan,Erica J Nelson,Mohammad Akhshik,George C Privon,Qi Li,Francesco Valentino

doi:10.3847/2041-8213/ac399f

Abstract

Abstract Observations of cold molecular gas reservoirs are critical for understanding the shutdown of star formation in massive galaxies. While dust continuum is an efficient and affordable tracer, this method relies upon the assumption of a “normal” molecular-gas to dust mass ratio, δ GDR, typically of order 100. Recent null detections of quiescent galaxies in deep dust continuum observations support a picture where the cold gas and dust have been rapidly depleted or expelled. In this work, we present another viable explanation: a significant fraction of galaxies with low star formation per unit stellar mass are predicted to have extreme δ GDR ratios. We show that simulated massive quiescent galaxies at 0 < z < 3 in the simba cosmological simulations have δ GDR values that extend >4 orders of magnitude. The dust in most simulated quiescent galaxies is destroyed significantly more rapidly than the molecular gas depletes, and cannot be replenished. The transition from star-forming to quiescent halts dust formation via star formation processes, with dust subsequently destroyed by supernova shocks and thermal sputtering of dust grains embedded in hot plasma. After this point, the dust growth rate in the models is not sufficient to overcome the loss of >3 orders of magnitude in dust mass to return to normal values of δ GDR despite having high metallicity. Our results indicate that it is not straight forward to use a single observational indicator to robustly preselect exotic versus normal ratios. These simulations make strong predictions that can be tested with millimeter facilities.

Highlights

We select a total of 17,869 simulated galaxies having log(Må/Me) > 10, identified via a 6D friends-of-friends algorithm applied to dense gas and stars in four snapshots (z = 0, 1, 2, 3) of the (100 h−1 Mpc)3 SIMBA simulation
In this Letter, we present predictions from the hydrodynamic cosmological SIMBA simulations showing there is a dramatic increase in the molecular-gas mass to dust mass ratios, δGDR, when star formation slows and falls below log(sSFR) −10 yr−1
The wide range of predicted δGDR values within the simulations cannot be explained by variations in metallicity as almost all model galaxies at low sSFR have solar or supersolar metallicities

Summary

Introduction

After over a decade and thousands of hours of observations, we have a reasonably good census of the dust and cold gas content in normal star-forming galaxies out to z ∼ 2, as well as other galaxy properties that correlate (see reviews by, e.g., Carilli & Walter 2013; Hodge & da Cunha 2020; Tacconi et al 2020). The dust and cold gas content of quiescent galaxies remain uncertain, especially toward higher redshift. Spectroscopic studies measuring the cold gas content of quiescent galaxies at z > 0.5 are limited, as robust constraints are observationally expensive. The diversity in MH2 may be due to the range in specific star formation rates (sSFR ≡ SFR/Må) of the samples themselves, as low sSFRs are hard to constrain (e.g., Leja et al 2019)

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