The number density and mass density of star-forming and quiescent
  galaxies at 0.4 < z < 2.2
  We study the build-up of the bimodal galaxy population using the NEWFIRM
Medium-Band Survey, which provides excellent redshifts and well-sampled
spectral energy distributions of ~27,000 galaxies with K<22.8 at 0.4 < z < 2.2.
We first show that star-forming galaxies and quiescent galaxies can be robustly
separated with a two-color criterion over this entire redshift range. We then
study the evolution of the number density and mass density of quiescent and
star-forming galaxies, extending the results of the COMBO-17, DEEP2, and other
surveys to z=2.2. The mass density of quiescent galaxies with M > 3 10^11 solar
masses increases by a factor of ~10 from z=2 to the present day, whereas the
mass density in star-forming galaxies is flat or decreases over the same time
period. Modest mass growth by a factor of 2 of individual quiescent galaxies
can explain roughly half of the strong density evolution at M>10^11 solar
masses, due to the steepness of the exponential tail of the mass function. The
rest of the density evolution of massive, quiescent galaxies is likely due to
transformation (e.g. quenching) of the massive star-forming population, a
conclusion which is consistent with the density evolution we observe for the
star-forming galaxies themselves, which is flat or decreasing with cosmic time.
Modest mass growth does not explain the evolution of less massive quiescent
galaxies (~10^10.5 solar masses), which show a similarly steep increase in
their number densities. The less massive quiescent galaxies are therefore
continuously formed by transforming galaxies from the star-forming population
