MORPHOLOGICAL QUENCHING OF STAR FORMATION: MAKING EARLY-TYPE GALAXIES RED

Abstract

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