  We show that the cool gas masses of galactic discs reach a steady state that
lasts many Gyr after their last major merger in cosmological hydrodynamic
simulations. The mass of disc gas, M$_{\rm gas}$, depends upon a galaxy halo's
spin and virial mass, but not upon stellar feedback. Halos with low spin have
high star formation efficiency and lower disc gas mass. Similarly, lower
stellar feedback leads to more star formation so the gas mass ends up nearly
the same irregardless of stellar feedback strength. Even considering spin, the
M$_{\rm gas}$ relation with halo mass, M$_{200}$ only shows a factor of 3
scatter. The M$_{\rm gas}$--M$_{200}$ relation show a break at
M$_{200}$=$2\times10^{11}$ M$_\odot$ that corresponds to an observed break in
the M$_{\rm gas}$--M$_\star$ relation. The constant disc mass stems from a
shared halo gas density profile in all the simulated galaxies. In their outer
regions, the profiles are isothermal. Where the profile rises above $n=10^{-3}$
cm$^{-3}$, the gas readily cools and the profile steepens. Inside the disc,
rotation supports gas with a flatter density profile except where supernova
explosions disrupt the disc. Energy injection from stellar feedback also
provides pressure support to the halo gas to prevent runaway cooling flows. The
resulting constant gas mass makes simpler models for galaxy formation possible,
either using a "bathtub" model for star formation rates or when modeling
chemical evolution.
