Connecting and dissecting galaxies' angular momenta and neutral gas in a
  hierarchical universe: cue DARK SAGE
  We explore the connection between the atomic gas fraction, f_atm, and `global
disc stability' parameter, q, of galaxies within a fully cosmological context
by examining galaxies in the Dark Sage semi-analytic model. The q parameter is
determined by the ratio of disc specific angular momentum to mass. Dark Sage is
well suited to our study, as it includes the numerical evolution of
one-dimensional disc structure, making both j_disc and q predicted quantities.
We show that Dark Sage produces a clear correlation between gas fraction and
j_disc at fixed disc mass, in line with recent results from observations and
hydrodynamic simulations. This translates to a tight q--f_atm sequence for
star-forming central galaxies, which closely tracks the analytic prediction of
Obreschkow et al. The scatter in this sequence is driven by the probability
distribution function of mass as a function of j (PDF of j) within discs,
specifically where it peaks. We find that halo mass is primarily responsible
for the peak location of the PDF of j, at least for low values of q. Two main
mechanisms of equal significance are then identified for disconnecting f_atm
from q. Mergers in the model can trigger quasar winds, with the potential to
blow out most of the gas disc, while leaving the stellar disc relatively
unharmed. Ram-pressure stripping of satellite galaxies has a similar effect,
where f_atm can drop drastically with only a minimal effect to q. We highlight
challenges associated with following these predictions up with observations.
