2009AJ....138..579K

We identify a population of morphologically defined E/S0 galaxies lying on the locus of late-type galaxies in color-stellar mass space–the "blue sequence"–at the present epoch. Using three samples (from the Nearby Field Galaxy Survey or NFGS, a merged HyperLeda/Sloan Digital Sky Survey/Two Micron All Sky Survey catalog, and the NYU Value-Added Galaxy Catalog), we analyze blue-sequence E/S0s with stellar masses ≳10⁸ M_☉, arguing that individual objects may be evolving either up toward the red sequence or down into the blue sequence. Blue-sequence E/S0 galaxies become more common with decreasing stellar mass, comprising ≲2% of E/S0s near the "shutdown mass" M_s∼ 1-2x10¹¹ M_☉, increasing to ≳5% near the "bimodality mass" M_b∼ 3x10¹⁰ M_☉, and sharply rising to ≳ 20%-30% below the "threshold mass" M_t∼ 4-6x10⁹ M_☉, down to our completeness analysis limit at ∼10⁹ M_☉. The strong emergence of blue-sequence E/S0s below M_t coincides with a previously reported global increase in mean atomic gas fractions below M_t for galaxies of all types on both sequences, suggesting that the availability of cold gas may be basic to blue-sequence E/S0s' existence. Environmental analysis reveals that many sub-M_bblue-sequence E/S0s reside in low-to-intermediate density environments. Thus, the bulk of the population we analyze appears distinct from the generally lower-mass cluster dE population; S0 morphologies with a range of bulge sizes are typical. In mass-radius and mass-σ scaling relations, blue-sequence E/S0s are more similar to red-sequence E/S0s than to late-type galaxies, but they represent a transitional class. While some of them, especially in the high-mass range from M_b to M_s, resemble major-merger remnants that will likely fade onto the red sequence, most blue-sequence E/S0s below M_b show signs of disk and/or pseudobulge building, which may be enhanced by companion interactions. The blue overall colors of blue-sequence E/S0s are most clearly linked to blue outer disks, but also reflect blue centers and more frequent blue-centered color gradients than seen in red-sequence E/S0s. Notably, all E/S0s in the NFGS with polar or counterrotating gas lie on or near the blue sequence, and most of these systems show signs of secondary stellar disks forming in the decoupled gas. From star formation rates and gas fractions, we infer significant recent and ongoing morphological transformation in the blue-sequence E/S0 population, especially below M_b. We argue that sub-M_bblue-sequence E/S0s occupy a "sweet spot" in stellar mass and concentration, with both abundant gas and optimally efficient star formation, which may enable the formation of large spiral disks. Our results provide evidence for the importance of disk rebuilding after mergers, as predicted by hierarchical models of galaxy formation.