The evolution of the scatter of the cosmic average color-magnitude
  relation: Demonstrating consistency with the ongoing formation of elliptical
  galaxies
  We present first measurements of the evolution of the scatter of the cosmic
average early-type galaxy color-magnitude relation (CMR) from z=1 to the
present day, finding that it is consistent with models in which galaxies are
constantly being added to the red sequence through truncation of star formation
in blue cloud galaxies. We used a sample of over 700 red sequence,
structurally-selected early-type galaxies (defined to have Sersic index >2.5)
with redshifts 0<z<1 taken from the Extended Chandra Deep Field South (173
galaxies) and the Sloan Digital Sky Survey (550 galaxies), constructing
rest-frame U-V colors accurate to <0.04mag. We find that the scatter of the CMR
of cosmic average early-type galaxies is ~0.1mag in rest-frame U-V color at
0.05<z<0.75, and somewhat higher at z=1. We compared these observations with a
model in which new red sequence galaxies are being constantly added at the rate
required to match the observed number density evolution, and found that this
model predicts the correct CMR scatter and its evolution. Furthermore, this
model predicts approximately the correct number density of `blue spheroids' -
structurally early-type galaxies with blue colors - albeit with considerable
model dependence. Thus, we conclude that both the evolution of the number
density and colors of the early-type galaxy population paint a consistent
picture in which the early-type galaxy population grows significantly between
z=1 and the present day through the quenching of star formation in blue cloud
galaxies.
