Morphological quenching of star formation: making early-type galaxies
  red
  We point out a natural mechanism for quenching of star formation in
early-type galaxies. It automatically links the color of a galaxy with its
morphology and does not require gas consumption, removal or termination of gas
supply. Given that star formation takes place in gravitationally unstable gas
disks, it can be quenched when a disk becomes stable against fragmentation to
bound clumps. This can result from the growth of a stellar spheroid, for
instance by mergers. We present the concept of morphological quenching (MQ)
using standard disk instability analysis, and demonstrate its natural
occurrence in a cosmological simulation using an efficient zoom-in technique.
We show that the transition from a stellar disk to a spheroid can be sufficient
to stabilize the gas disk, quench star formation, and turn an early-type galaxy
red and dead while gas accretion continues. The turbulence necessary for disk
stability can be stirred up by sheared perturbations within the disk in the
absence of bound star-forming clumps. While gas stripping processes are limited
to dense groups and clusters, and other quenching mechanisms like AGN feedback,
virial shock heating and gravitational heating, are limited to halos more
massive than 10^12 Mo, the MQ can explain the appearance of red ellipticals
even in less massive halos and in the field. The dense gas disks observed in
some of today's red ellipticals may be the relics of this mechanism, whereas
red galaxies with quenched gas disks are expected to be more frequent at high
redshift.
