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 } M _ { \odot } ) 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 ) \sim 2 % , and rest-frame colors , \sigma _ { \mathrm { U - V } } \sim 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 4000 \mathrm { \AA } - 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 \lesssim 1 , and a frosting of relatively young stars from star formation at later times .