The Interstellar Medium Of Quiescent Galaxies And Its Evolution With Time

We characterise the basic far-IR properties and the gas mass fraction of massive (log(M-*/M-circle dot)approximate to 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-*/M-circle dot)greater than or similar to 10.8 QGs. We find that the dust to stellar mass ratio (M-dust/M-*) rises steeply as a function of redshift up to z similar to 1.0 and then remains flat at least out to z=2.0. Using M-dust as a proxy of gas mass (M-gas), we find a similar trend for the evolution of the gas mass fraction (f(gas)), with z>1.0 QGs having f(gas)approximate to 7.0% (for solar metallicity). This f(gas) is three to ten times lower than that of normal star-forming galaxies (SFGs) at their corresponding redshift but greater than or similar to 3 and greater than or similar to 10 times larger compared to that of z=0.5 and local QGs. Furthermore, the inferred gas depletion time scales are comparable to those of local SFGs and systematically longer than those of main sequence galaxies at their corresponding redshifts. Our analysis also reveals that the average dust temperature (T-d) of massive QGs remains roughly constant (T-d = 21.0 +/- 2.0 K) at least out to z approximate to 2.0 and is substantially colder (Delta T-d approximate to 10 K) compared to that of SFGs. This motivated us to construct and release a redshift-invariant template IR SED, that we used to make predictions for ALMA observations and to explore systematic effects in the M-gas estimates of massive, high-z QGs. Finally, we discuss how a simple model that considers progenitor bias can effectively reproduce the observed evolution of M-dust/M-* and f(gas). Our results indicate universal initial interstellar medium conditions for quenched galaxies and a large degree of uniformity in their internal processes across cosmic time.