The Star Formation Histories of Disk Galaxies: the Live, the Dead, and
  the Undead
  We reexamine the systematic properties of local galaxy populations, using
published surveys of star formation, structure, and gas content. After
recalibrating star formation measures, we are able to reliably measure specific
star formation rates well below the "main sequence" of star formation vs mass.
We find an unexpectedly large population of galaxies with star formation rates
intermediate between vigorously star-forming main sequence galaxies and passive
galaxies, and with gas content disproportionately high for their star formation
rates. Several lines of evidence suggest that these quiescent galaxies form a
distinct population rather than a low star formation tail of the main sequence.
We demonstrate that a tight main sequence, evolving with epoch, is a natural
outcome of most histories of star formation and has little astrophysical
significance, but that the quiescent population requires additional
astrophysics to explain its properties. Using a simple model for disk evolution
based on the observed dependence of star formation on gas content in local
galaxies, and assuming simple histories of cold gas inflow, we show that the
evolution of galaxies away from the main sequence can be attributed to the
depletion of gas due to star formation after a cutoff in gas inflow. The
quiescent population is composed of galaxies in which the density of disk gas
has fallen below a threshold for disk stability. The evolution of galaxies
beyond the quiescent state to gas exhaustion requires another process, probably
wind-driven mass loss. The SSFR distribution of the quiescent and passive
implies that the timescale of this process must be greater than a few Gyrs but
less than a few tens of Gyrs. The environmental dependence of the galaxy
populations is consistent with recent theory suggesting that cold gas inflows
into galaxies are truncated at earlier times in denser environments.
