The Effect of Dry Mergers on the Color-Magnitude Relation
  We investigate the effect of gas-poor (so-called "dry") mergers on the
color-magnitude relation (CMR) of early-type galaxies through a simple toy
model and compare with low-z observations from the Sloan Digital Sky Survey
(SDSS). The observed red sequence shows a tilt towards bluer colors and a
decrease in scatter at the bright end. These characteristics are predicted by
our model, based on merger trees from a semi-analytic model of galaxy
formation. We assume galaxies move onto a "creation red sequence" when they
undergo major gas-rich mergers. Subsequent dry mergers move galaxies along the
relation by increasing their mass, but also make them slightly bluer. This
occurs because bright galaxies are most likely to merge with one of the more
numerous fainter and consequently bluer galaxies that lie further down the
relation. Bright galaxies undergo a higher fraction of dry mergers than faint
galaxies, which causes a change in the slope of the CMR. A more realistic model
that includes scatter in the initial relation shows that dry merging causes a
tightening of the CMR towards the bright end. The small scatter in the observed
CMR thus cannot be used to argue against significant mass growth from dry
merging.
