H_160_-selected catalog of galaxies in the HUDF

The observed SFRs can fully account for the mass growth to z~2 among galaxies with cumulative number densities greater than 10^-3.5^Mpc^-3^. For galaxies with a lower constant number density (higher mean mass), we find the observed stellar masses are ~three times greater than that which may be accounted for by the observed star formation alone at late times, implying that growth from mergers plays an important role at z<2.

We additionally observe a decreasing sSFR, equivalent to approximately one order of magnitude, from z~6 to z~2 among galaxies with number densities less than 10^-3.5^Mpc^3^, along with significant evidence that at any redshift the sSFR is higher for galaxies at higher number density. The combination of these findings can qualitatively explain the previous findings of a specific star formation rate plateau at high redshift.

Tracing the evolution of the fraction of quiescent galaxies for samples matched in cumulative number density over this redshift range, we find no unambiguous examples of quiescent galaxies at z>4.