Observational Evidence for the Co-evolution of Galaxy Mergers, Quasars, and the Blue/red Galaxy Transition

Observations indicate the mass above which ellipticals dominate the galaxy population increases with redshift over the range 0 < z < 2, implying that galaxies transition from the blue to red sequence in a manner consistent with “cosmic downsizing.” Theories predict that mergers between gas-rich blue galaxies produce ellipticals, with an associated phase of bright quasar activity which expels and heats gas, terminating star formation, so that the remnant rapidly becomes red. If this is true, then the transition mass at a given redshift, where the red sequence is being built, should also correspond to the characteristic mass of both merging gasrich galaxies and quasar hosts at that redshift. We compare observations of these quantities from 0 < z < 3, as well as theoretical models of mergers and associated quasar activity, and show that they indeed appear to be the same mass and evolve in the same manner over this range. We also provide new tests of this association, such as the AGN fraction peaking near the transition mass and the quasar host mass as a function of luminosity. Our results support the view that mergers and subsequent quasar activity drive the transition of galaxies from the blue to the red sequence. Subject headings: quasars: general — galaxies: active — galaxies: evolution — cosmology: theory