Dry mergers in gems:: The dynamical evolution of massive early-type galaxies

We have used the 28' x 28' Hubble Space Telescope image mosaic from the GEMS (Galaxy Evolution from Morphology and SEDs) survey in conjunction with the COMBO-17 photometric redshift survey to constrain the incidence of major mergers between spheroid-dominated galaxies with little cold gas (dry mergers) since z = 0.7. A set of N-body merger simulations was used to explore the morphological signatures of such interactions: they are recognizable either as < 5 kpc separation close pairs or because of broad, low surface brightness tidal features and asymmetries. Data with the depth and resolution of GEMS are sensitive to dry mergers between galaxies with M-V less than or similar to -20.5 for z less than or similar to 0.7; dry mergers at higher redshifts are not easily recovered in single-orbit HST imaging. Six dry mergers ( 12 galaxies) with luminosity ratios between 1 : 1 and 4: 1 were found from a sample of 379 red early-type galaxies with MV < -20.5 and 0.1 < z < 0.7. The simulations suggest that the morphological signatures of dry merging are visible for similar to 150 Myr, and we use this timescale to convert the observed merger incidence into a rate. On this basis we find that present-day spheroidal galaxies with M-V < -20.5 on average have undergone between 0.5 and 2 major dry mergers since z similar to 0.7. We have compared this result with the predictions of a cold dark matter-based semianalytic galaxy formation model. The model reproduces the observed declining major merger fraction of bright galaxies and the space density of luminous early-type galaxies reasonably well. The predicted dry merger fraction is consistent with our observational result. Hence, hierarchical models predict and observations now show that major dry mergers are an important driver of the evolution of massive early-type galaxies in recent epochs.