From Blue Star-forming to Red Passive: Galaxies in Transition in Different Environments

Abstract

Exploiting a mass-complete (M <SUB>*</SUB> > 10<SUP>10.25</SUP> M <SUB>☉</SUB>) sample at 0.03 <z < 0.11 drawn from the Padova Millennium Galaxy Group Catalog, we use the (U - B)<SUB> rf </SUB> color and morphologies to characterize galaxies, in particular those that show signs of an ongoing or recent transformation of their star-formation activity and/or morphology: green galaxies, red passive late types, and blue star-forming early types. Color fractions depend on mass and only for M <SUB>*</SUB> < 10<SUP>10.7</SUP> M <SUB>☉</SUB> on environment. The incidence of red galaxies increases with increasing mass, and, for M <SUB>*</SUB> < 10<SUP>10.7</SUP> M <SUB>☉</SUB>, decreases toward the group outskirts and in binary and single galaxies. The relative abundance of green and blue galaxies is independent of environment and increases monotonically with galaxy mass. We also inspect galaxy structural parameters, star-formation properties, histories, and ages and propose an evolutionary scenario for the different subpopulations. Color transformations are due to a reduction and suppression of the star-formation rate in both bulges and disks that does not noticeably affect galaxy structure. Morphological transitions are linked to an enhanced bulge-to-disk ratio that is due to the removal of the disk, not to an increase of the bulge. Our modeling suggests that green colors might be due to star-formation histories declining with long timescales, as an alternative scenario to the classical "quenching" processes. Our results suggest that galaxy transformations in star-formation activity and morphology depend neither on the environment nor on being a satellite or the most massive galaxy of a halo. The only environmental dependence we find is the higher fast quenching efficiency in groups giving origin to poststarburst signatures.