The effect of dry mergers on the color-magnitude relation of early-type
  galaxies
  We investigate the effect of dry merging on the color-magnitude relation
(CMR) of galaxies and find that the amount of merging predicted by a
hierarchical model results in a red sequence that compares well with the
observed low-redshift relation. A sample of 29,000 early-type galaxies selected
from the Sloan Digital Sky Survey Data Release 6 shows that the bright end of
the CMR has a shallower slope and smaller scatter than the faint end. This
magnitude dependence is predicted by a simple toy model in which gas-rich
mergers move galaxies onto a "creation red sequence" (CRS) by quenching their
star formation, and subsequent mergers between red, gas-poor galaxies
(so-called "dry" mergers) move galaxies along the relation. We use galaxy
merger trees from a semianalytic model of galaxy formation to test the
amplitude of this effect and find a change in slope at the bright end that
brackets the observations, using gas fraction thresholds of 10-30% to separate
wet and dry mergers. A more realistic model that includes scatter in the CRS
shows that dry merging decreases the scatter at the bright end. Contrary to
previous claims, the small scatter in the observed CMR thus cannot be used to
constrain the amount of dry merging.
