Abstract
We use the SDSS to explore early-type galaxies (ETGs) that are plausibly in
transition between the red sequence and the blue cloud. Through careful
morphological inspection, we identify 1500 unusually blue elliptical galaxies
among a larger sample of blue ETGs with 0.01<z<0.08 and M_star>1e10 h^-2
M_sun. Blue ellipticals comprise 3.7% of all ETGs with 1e10<M_star<1e11 h^-2
M_sun. Using fiber spectra diagnostics, we find a unique population of 172
non-star-forming ellipticals with distinctly blue urz colors and <3Gyr stellar
ages. These recently quenched ellipticals (RQEs) have n=2.7-4.7x10^-5h^3
Mpc^-3 and sufficient numbers above 2.5e10 h^-2 M_sun to account for >50%
of the expected quiescent growth at z~0 assuming this phase lasts 0.5Gyr. Based
on color, stellar M/L ratios and metallicity estimates, we argue that RQEs have
recent SFHs that differ from rejuvenated ETGs. Yet, a significant but small
incidence (5%) of RQEs with E+A spectra implies that the quenching of most RQEs
did not involve a large starburst. Most (90%) RQEs reside at the centers of
3x10^12h^-1 M_sun groups, which agrees well with the `small group scale' in
which spiral merging onto the halo center is maximally efficient. The preferred
RQE environment rules out satellite-specific quenching processes for most.
Under the assumption that most RQEs were quenched by a hot halo atmosphere
impeding efficient gas cooling, we discuss the incidence of Seyfert and LINER
activity with regards to the maintenance-mode feedback needed to keep star
formation shut off at the centers of such small halos. (abridged)
Users
Please
log in to take part in the discussion (add own reviews or comments).