The Age Spread of Quiescent Galaxies with the NEWFIRM Medium-band
  Survey: Identification of the Oldest Galaxies out to z~2
  With a complete, mass-selected sample of quiescent galaxies from the NEWFIRM
Medium-Band Survey (NMBS), we study the stellar populations of the oldest and
most massive galaxies (>10^11 Msun) to high redshift. The sample includes 570
quiescent galaxies selected based on their extinction-corrected U-V colors out
to z=2.2, with accurate photometric redshifts, sigma_z/(1+z)~2%, and rest-frame
colors, sigma_U-V~0.06 mag. We measure an increase in the intrinsic scatter of
the rest-frame U-V colors of quiescent galaxies with redshift. This scatter in
color arises from the spread in ages of the quiescent galaxies, where we see
both relatively quiescent red, old galaxies and quiescent blue, younger
galaxies towards higher redshift. The trends between color and age are
consistent with the observed composite rest-frame spectral energy distributions
(SEDs) of these galaxies. The composite SEDs of the reddest and bluest
quiescent galaxies are fundamentally different, with remarkably well-defined
4000A- and Balmer-breaks, respectively. Some of the quiescent galaxies may be
up to 4 times older than the average age- and up to the age of the universe, if
the assumption of solar metallicity is correct. By matching the scatter
predicted by models that include growth of the red sequence by the
transformation of blue galaxies to the observed intrinsic scatter, the data
indicate that most early-type galaxies formed their stars at high redshift with
a burst of star formation prior to migrating to the red sequence. The observed
U-V color evolution with redshift is weaker than passive evolution predicts;
possible mechanisms to slow the color evolution include increasing amounts of
dust in quiescent galaxies towards higher redshift, red mergers at z<1, and a
frosting of relatively young stars from star formation at later times.
