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 (z ≤ 0.08) that have unusually blue optical colours. Through careful inspection, we distinguish elliptical from non-elliptical morphologies among a large sample of similarly blue galaxies with high central light concentrations (c_r ≥ 2.6). These blue ellipticals comprise 3.7 per cent of all c_r ≥ 2.6 galaxies with stellar masses between 10¹⁰ and 10¹¹ h^-2 M_⊙. Using published fibre spectrum diagnostics, we identify a unique subset of 172 non-star-forming ellipticals with distinctly blue urz colours and young (<3 Gyr) light-weighted stellar ages. These recently quenched ellipticals (RQEs) have a number density of 2.7-4.7 × 10^{- 5} h³ Mpc^{- 3} and sufficient numbers above 2.5 × 10¹⁰ h^-2 M_⊙ to account for more than half of the expected quiescent growth at late cosmic time assuming that 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 centres of <3 × 10<sup>12</sup> h<sup>-1</sup> M<sub>⊙</sub> groups) agrees well with the `small group scale' predicted for maximally efficient spiral merging on to their halo centre and rules out satellite-specific quenching processes. The high incidence of Seyfert and LINER activity in RQEs and their plausible descendants may heat the atmospheres of small host haloes sufficiently to maintain quenching.