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 M⊙, 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.