How is Star Formation Quenched in Massive Galaxies?
  The bimodality in observed present-day galaxy colours has long been a
challenge for hierarchical galaxy formation models, as it requires some
physical process to quench (and keep quenched) star formation in massive
galaxies. Here we examine phenomenological models of quenching by
post-processing the star formation histories of galaxies from cosmological
hydrodynamic simulations that reproduce observations of star-forming galaxies
reasonably well. We consider recipes for quenching based on major mergers, halo
mass thresholds, gas temperature thresholds, and variants thereof. We compare
the resulting simulated star formation histories to observed g-r
colour-magnitude diagrams and red and blue luminosity functions from SDSS. The
merger and halo mass quenching scenarios each yield a distinct red sequence and
blue cloud of galaxies that are in broad agreement with data, albeit only under
rather extreme assumptions. In detail, however, the simulated red sequence
slope and amplitude in each scenario is somewhat discrepant, perhaps traceable
to low metallicities in simulated galaxies. Merger quenching produces more
massive blue galaxies, earlier quenching, and more frosting of young stars;
comparing to relevant data tends to favor merger over halo mass quenching.
Although physically-motivated quenching models can produce a red sequence,
interesting generic discrepancies remain that indicate that additional physics
is required to reproduce the star formation and enrichment histories of red and
dead galaxies.
