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We reexamine the systematic properties of local galaxy populations, usingpublished surveys of star formation, structure, and gas content. Afterrecalibrating star formation measures, we are able to reliably measure specificstar formation rates well below the "main sequence" of star formation vs mass.We find an unexpectedly large population of galaxies with star formation ratesintermediate between vigorously star-forming main sequence galaxies and passivegalaxies, and with gas content disproportionately high for their star formationrates. Several lines of evidence suggest that these quiescent galaxies form adistinct population rather than a low star formation tail of the main sequence.We demonstrate that a tight main sequence, evolving with epoch, is a naturaloutcome of most histories of star formation and has little astrophysicalsignificance, but that the quiescent population requires additionalastrophysics to explain its properties. Using a simple model for disk evolutionbased on the observed dependence of star formation on gas content in localgalaxies, and assuming simple histories of cold gas inflow, we show that theevolution of galaxies away from the main sequence can be attributed to thedepletion of gas due to star formation after a cutoff in gas inflow. Thequiescent population is composed of galaxies in which the density of disk gashas fallen below a threshold for disk stability. The evolution of galaxiesbeyond the quiescent state to gas exhaustion requires another process, probablywind-driven mass loss. The SSFR distribution of the quiescent and passiveimplies that the timescale of this process must be greater than a few Gyrs butless than a few tens of Gyrs. The environmental dependence of the galaxypopulations is consistent with recent theory suggesting that cold gas inflowsinto galaxies are truncated at earlier times in denser environments.

The Star Formation Histories of Disk Galaxies: the Live, the Dead, and the Undead