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 (<log(M*/Msun)> ~ 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 (< Td > = 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.
