Modeling the red sequence: Hierarchical growth yet slow luminosity
  evolution
  We explore the effects of mergers on the evolution of massive early-type
galaxies by modeling the evolution of their stellar populations in a
hierarchical context. We investigate how a realistic red sequence population
set up by z~1 evolves under different assumptions for the merger and star
formation histories, comparing changes in color, luminosity and mass. The
purely passive fading of existing red sequence galaxies, with no further
mergers or star formation, results in dramatic changes at the bright end of the
luminosity function and color-magnitude relation. Without mergers there is too
much evolution in luminosity at a fixed space density compared to observations.
The change in color and magnitude at a fixed mass resemble that of a passively
evolving population that formed relatively recently, at z~2. Mergers amongst
the red sequence population ("dry mergers") occurring after z=1 build up mass,
counteracting the fading of the existing stellar populations to give smaller
changes in both color and luminosity for massive galaxies. By allowing some
galaxies to migrate from the blue cloud onto the red sequence after z=1 through
gas-rich mergers, younger stellar populations are added to the red sequence.
This manifestation of the progenitor bias increases the scatter in age and
results in even smaller changes in color and luminosity between z=1 and z=0 at
a fixed mass. The resultant evolution appears much slower, resembling the
passive evolution of a population that formed at high redshift (z~3-5) and is
in closer agreement with observations. Measurements of the luminosity and color
evolution alone are not sufficient to distinguish between the purely passive
evolution of an old population and cosmologically motivated hierarchical
growth, although these scenarios have very different implications for the mass
growth of early-type galaxies over the last half of cosmic history.
