Intrinsic colors and ages of extremely red elliptical galaxies at high
  redshift
  In order to know the formation epoch of the oldest elliptical galaxies as a
function of mass and observed redshift, a statistical analysis for 333
extremely red objects (EROs) classified as old galaxies (OGs) at 0.8<z<2.3 is
carried out. Once we get M_V and (B-V) at rest for each galaxy, we calculate
the average variation of this intrinsic color with redshift and derive the
average age through a synthesis model (the code for the calculation of the age
has been made publicly available). The average gradient of the (B-V) color at
rest of EROs/OGs is 0.07-0.10 Gyr^{-1} for a fixed luminosity. The stars in
these extremely red elliptical galaxies were formed when the Universe was ~2
Gyr old on average. We have not found a significant enough dependence on the
observed redshift and stellar mass: dt_{formation}/dt_{observed}=-0.46+/-0.32,
dt_{formation}/(d log_10 M_*)=-0.81+/-0.98 Gyr. This fits a scenario in which
the stellar formation of the objects that we denominate as EROs-OGs is more
intense at higher redshifts, at which the stellar populations of the most
massive galaxies form earlier than or at the same time as less massive
galaxies.
