The evolution of quiescent galaxies at high redshift (z > 1.4)
  We have studied the evolution of high redshift quiescent galaxies over an
effective area of ~1.7 deg^2 in the COSMOS field. Galaxies have been divided
according to their star-formation activity and the evolution of the different
populations has been investigated in detail. We have studied an IRAC (mag_3.6 <
22.0) selected sample of ~18000 galaxies at z > 1.4 with multi-wavelength
coverage. We have derived accurate photometric redshifts (sigma=0.06) and other
important physical parameters through a SED-fitting procedure. We have divided
our sample into actively star-forming, intermediate and quiescent galaxies
depending on their specific star formation rate. We have computed the galaxy
stellar mass function of the total sample and the different populations at
z=1.4-3.0. We have studied the properties of high redshift quiescent galaxies
finding that they are old (1-4 Gyr), massive (log(M/M_sun)~10.65), weakly star
forming stellar populations with low dust extinction (E(B-V) < 0.15) and small
e-folding time scales (tau ~ 0.1-0.3 Gyr). We observe a significant evolution
of the quiescent stellar mass function from 2.5 < z < 3.0 to 1.4 < z < 1.6,
increasing by ~ 1 dex in this redshift interval. We find that z ~ 1.5 is an
epoch of transition of the GSMF. The fraction of star-forming galaxies
decreases from 60% to 20% from z ~ 2.5-3.0 to z ~ 1.4-1.6 for log(M/M_sun) >
11, while the quiescent population increases from 10% to 50% at the same
redshift and mass intervals. We compare the fraction of quiescent galaxies
derived with that predicted by theoretical models and find that the Kitzbichler
& White (2007) model is the one that better reproduces the data. Finally, we
calculate the stellar mass density of the star-forming and quiescent
populations finding that there is already a significant number of quiescent
galaxies at z > 2.5 (rho~6.0 MsunMpc^-3).
