COLOR DISTRIBUTIONS, NUMBER, AND MASS DENSITIES OF MASSIVE GALAXIES AT 1.5 < z < 3: COMPARING OBSERVATIONS WITH MERGER SIMULATIONS

We present a comparison between the observed color distribution, number, and mass density of massive galaxies at 1.5 < z < 3 and a model by Hopkins et al. that relates the quasar and galaxy population on the basis of gas-rich mergers. In order to test the hypothesis that quiescent red galaxies are formed after a gas-rich merger involving quasar activity, we confront photometry of massive (M>4 x 1010 Msun) galaxies extracted from the FIRES, GOODS-South, and MUSYC surveys, together spanning an area of 496 arcmin2, with synthetic photometry from hydrodynamical merger simulations. As in the Hopkins et al. model, we use the observed quasar luminosity function to estimate the merger rate. We find that the synthetic U - V and V - J colors of galaxies that had a quasar phase in their past match the colors of observed galaxies that are best characterized by a quiescent stellar population. At z ∼ 2.6, the observed number and mass density of quiescent red galaxies with M>4 x 1010 Msun is consistent with the model in which every quiescent massive galaxy underwent a quasar phase in the past. At z ∼ 1.9, 2.8 times less quiescent galaxies are observed than predicted by the model as descendants of higher redshift quasars. The merger model also predicts a large number and mass density of galaxies undergoing star formation driven by the merger. We find that the predicted number and mass density accounts for 30%-50% of the observed massive star-forming galaxies. However, their colors do not match those of observed star-forming galaxies. In particular, the colors of dusty red galaxies (accounting for 30%-40% of the massive galaxy population) are not reproduced by the simulations. Several possible origins of this discrepancy are discussed. The observational constraints on the validity of the model are currently limited by cosmic variance and uncertainties in stellar population synthesis and radiative transfer.