A new population of recently quenched elliptical galaxies in the SDSS

We use the Sloan Digital Sky Survey to investigate the properties of massive elliptical galaxies in the local Universe (zleq0.08) that have unusually blue optical colors. Through careful inspection, we distinguish elliptical from non-elliptical morphologies among a large sample of similarly blue galaxies with high central light concentrations (c_rgeq2.6). These blue ellipticals comprise 3.7 per cent of all c_rgeq2.6 galaxies with stellar masses between 10^10 and 10^11 h^{-2} {rm M}_{sun}. Using published fiber spectra diagnostics, we identify a unique subset of 172 non-star-forming ellipticals with distinctly blue urz colors and young (< 3 Gyr) light-weighted stellar ages. These recently quenched ellipticals (RQEs) have a number density of 2.7-4.7times 10^{-5},h^3,{rm Mpc}^{-3} and sufficient numbers above 2.5times10^{10} h^{-2} {rm M}_{sun} to account for more than half of the expected quiescent growth at late cosmic time assuming this phase lasts 0.5 Gyr. RQEs have properties that are consistent with a recent merger origin (i.e., they are strong `first-generation elliptical candidates), yet few involved a starburst strong enough to produce an E+A signature. The preferred environment of RQEs (90 per cent reside at the centers of < 3times 10^{12},h^{-1}{rm M}_{sun} groups) agrees well with the `small group scale predicted for maximally efficient spiral merging onto their halo center and rules out satellite-specific quenching processes. The high incidence of Seyfert and LINER activity in RQEs and their plausible descendents may heat the atmospheres of small host halos sufficiently to maintain quenching.

Ongoing Assembly of Massive Galaxies by Major Merging in Large Groups and Clusters from the SDSS

We investigate the incidence of major mergers creating >10e11 Msun galaxies in present-day groups and clusters more massive than 2.5e13 Msun. We identify 38 pairs of massive galaxies with mutual tidal interaction signatures selected from >5000 galaxies with >5e10 Msun that reside in 845 such 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 asymmetries around each progenitor, such as off-center isophotes, broad tidal tails, and dynamical friction wakes. At the resolution and sensitivity of the SDSS, such mergers are found in 16% of high-mass, galaxy-galaxy pairs with magnitude differences of <1.5 and <30 kpc projected separations. We find that 90% of these mergers have nearly equal-mass progenitors with red-sequence colors and centrally-concentrated morphologies, the hallmarks of dissipationless merger simulations. Mergers at group centers are more common than between 2 satellites, but both are morphologically indistinguishable and we tentatively conclude that the latter are likely located at the dynamical centers of recently accreted subhalos. The frequency of central and satellite merging diminishes with group mass consistent with dynamical friction expectations. Based on reasonable assumptions, the centers of these massive halos are growing in stellar mass by 1-9% per Gyr, on average. Compared to all LRG-LRG mergers, we find a 2-9 times higher rate for their merging when restricted to these dense environments. Our results imply that the massive end of the galaxy population continues to evolve hierarchically at a measurable level, and that the centers of massive groups are the preferred environment for merger-driven galaxy assembly. (abridged)