The Interstellar Medium of Quiescent Galaxies and its Evolution With Time

We characterise the basic far-IR (FIR) properties and the gas mass fraction of massive ( ~ 11.0) quiescent galaxies (QGs) and explore how these evolve from z = 2.0 to the present day. We use robust, multi-wavelength (mid- to far-IR and sub-millimetre to radio) stacking ensembles of homogeneously selected and mass complete samples of log(M*/Msun) > 10.8 QGs. We find that the dust to stellar mass ratio (Md/M*) rises steeply as a function of redshift up to z~1.0 and then remains flat at least out to z = 2.0. Using Md as a proxy of gas mass (Mgas), we find a similar trend for the evolution of the gas mass fraction (fgas) with z > 1.0 QGs having fgas ~ 7.0% (for solar metallicity). This fgas is 3 - 10 times lower than that of normal star forming galaxies (SFGs) at their corresponding redshift but ~3 and ~10 times larger compared to that of z = 0.5 and local QGs. Furthermore, the inferred gas depletion time scales are comparable to that of local SFGs and systematically longer than that of main sequence galaxies at their corresponding redshifts. Our analysis also reveals that the average dust temperature (Td) of massive QGs remains roughly constant ( = 21.0 \pm 2.0K) at least out to z ~ 2.0 and is substantially colder (~ 10K) compared to that of z > 0 SFGs. This motivated us to construct and release a redshift-invariant template IR SED, that we use to make predictions for ALMA observations and to explore systematic effects in the Mgas estimates of massive, high-z QGs. Finally, we discuss how a simple model that considers progenitor-bias can effectively reproduce the observed evolution of Md/M* and fgas. Our results indicate universal initial interstellar medium conditions for quenched galaxies and a large degree of uniformity in their internal processes across cosmic time.