High Molecular-Gas to Dust Mass Ratios Predicted in Most Quiescent
  Galaxies
  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, $\delta_{\mathrm{GDR}}$, typically
of order one hundred. Recent null detections of quiescent galaxies in deep dust
continuum observations support a picture where the cold gas and dust has 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 $\delta_{\mathrm{GDR}}$ ratios.
We show that simulated massive quiescent galaxies at $0 < z < 3$ in the
\textsc{simba} cosmological simulations have $\delta_{\mathrm{GDR}}$ 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 $\delta_{\mathrm{GDR}}$ despite having
high metallicity. Our results indicate that it is not straight forward to use a
single observational indicator to robustly pre-select exotic versus normal
ratios. These simulations make strong predictions that can be tested with
millimeter facilities.
