The Green Valley is a Red Herring: Galaxy Zoo reveals two evolutionary
  pathways towards quenching of star formation in early- and late-type galaxies
  We use SDSS+\textit{GALEX}+Galaxy Zoo data to study the quenching of star
formation in low-redshift galaxies. We show that the green valley between the
blue cloud of star-forming galaxies and the red sequence of quiescent galaxies
in the colour-mass diagram is not a single transitional state through which
most blue galaxies evolve into red galaxies. Rather, an analysis that takes
morphology into account makes clear that only a small population of blue
early-type galaxies move rapidly across the green valley after the morphologies
are transformed from disk to spheroid and star formation is quenched rapidly.
In contrast, the majority of blue star-forming galaxies have significant disks,
and they retain their late-type morphologies as their star formation rates
decline very slowly. We summarize a range of observations that lead to these
conclusions, including UV-optical colours and halo masses, which both show a
striking dependence on morphological type. We interpret these results in terms
of the evolution of cosmic gas supply and gas reservoirs. We conclude that
late-type galaxies are consistent with a scenario where the cosmic supply of
gas is shut off, perhaps at a critical halo mass, followed by a slow exhaustion
of the remaining gas over several Gyr, driven by secular and/or environmental
processes. In contrast, early-type galaxies require a scenario where the gas
supply and gas reservoir are destroyed virtually instantaneously, with rapid
quenching accompanied by a morphological transformation from disk to spheroid.
This gas reservoir destruction could be the consequence of a major merger,
which in most cases transforms galaxies from disk to elliptical morphology, and
mergers could play a role in inducing black hole accretion and possibly AGN
feedback.
