Ongoing assembly of massive galaxies by major merging in large groups and clusters from the SDSS

We investigate the incidence of major mergers creating Mstar > 10M⊙ galaxies in the dense environments of present-day groups and clusters more massive than Mhalo = 2.5 × 10M⊙. We identify 38 pairs of massive galaxies with mutual tidal interaction signatures selected from > 5000 galaxies with Mstar ≥ 5×10M⊙ that reside in a halo mass-limited sample of 845 groups. We fit the images of each galaxy pair as the line-of-sight projection of symmetric models and identify mergers by the presence of residual asymmetric structure associated with both progenitors, such as nonconcentric isophotes, broad and diffuse tidal tails, and dynamical friction wakes. At the resolution and sensitivity of the SDSS, such mergers are found in 16% of the high-mass, galaxy-galaxy pairs with ≤ 1.5 r-band magnitude differences and ≤ 30 kpc projected separations. Relying on automated searches of major pairs from the SDSS spectroscopic galaxy sample will result in missing 70% of these mergers owing to spectroscopic incompleteness in high-density regions. We find that 90% of these mergers are between two nearly equal-mass progenitors with red-sequence colors and centrally-concentrated morphologies, in agreement with numerical simulations that predict that an important mechanism for the formation of massive elliptical galaxies is the dissipationless (gas-poor or so-called dry) major merging of spheroid-dominated galaxies. We identify seven additional Mstar > 10M⊙ mergers with disturbed morphologies and semi-resolved double nuclei. Mergers at the centers