Abstract
We relate transitions in galaxy structure and gas content to refueling, here
defined to include both the external gas accretion and the internal gas
processing needed to renew reservoirs for star formation. We analyze two z=0
data sets: a high-quality ~200-galaxy sample (the Nearby Field Galaxy Survey,
data release herein) and a volume-limited ~3000-galaxy sample with reprocessed
archival data. Both reach down to baryonic masses ~10^9Msun and span
void-to-cluster environments. Two mass-dependent transitions are evident: (i)
below the "gas-richness threshold" scale (V~125km/s), gas-dominated
quasi-bulgeless Sd--Im galaxies become numerically dominant, while (ii) above
the "bimodality" scale (V~200km/s), gas-starved E/S0s become the norm.
Notwithstanding these transitions, galaxy mass (or V as its proxy) is a poor
predictor of gas-to-stellar mass ratio M_gas/M_*. Instead, M_gas/M_* correlates
well with the ratio of a galaxy's stellar mass formed in the last Gyr to its
preexisting stellar mass, such that the two ratios have numerically similar
values. This striking correspondence between past-averaged star formation and
current gas richness implies routine refueling of star-forming galaxies on Gyr
timescales. We argue that this refueling underlies the tight M_gas/M_* vs.
color correlations often used to measure "photometric gas fractions."
Furthermore, the threshold and bimodality scale transitions reflect
mass-dependent demographic shifts between three refueling regimes --- accretion
dominated, processing dominated, and quenched. In this picture, gas-dominated
dwarfs are explained not by inefficient star formation but by overwhelming gas
accretion, which fuels stellar mass doubling in <~1Gyr. Moreover, moderately
gas-rich bulged disks such as the Milky Way are transitional, becoming abundant
only in the narrow range between the threshold and bimodality scales.