Niv Drory

Max-Planck-Institut für extraterrestrische Physik

The Galaxy Stellar Mass Function in COSMOS: New Results on an Old Galaxy Dichotomy

We present a new analysis of stellar mass functions (MFs) in the COSMOS field to fainter limits than has been previously probed at z < 1. Neither the total nor the red (passive) or blue (star-forming) MF can be well fit with a single Schechter function once one probes below 3e9 Msun. We observe a dip or plateau at masses ~1e10 Msun, and an upturn towards a steep faint-end slope of -1.7 at lower mass at all redshifts z < 1. This bimodal nature of the MF is not solely a result of the blue/red dichotomy. The blue MF is by itself bimodal at z ~ 1. This suggests a new dichotomy in galaxy formation that predates the appearance of the red sequence. We propose and discuss two interpretations for this bimodal mass distribution. If the gas fraction increases towards lower mass, galaxies with M_baryon ~ 1e10 Msun would shift to lower stellar masses, creating the observed dip. Alternatively, the dip could be created by an enhancement of the galaxy assembly rate at ~1e11 Msun, a phenomenon that naturally arises if the baryon fraction peaks at M_halo ~ 1e12 Msun. In this scenario, galaxies occupying the bump around M* would be identified with central galaxies, and the second fainter component having a steep faint-end slope with satellites. At the same time, we detect a drastic upturn in the number of low-mass red galaxies. Their increase with time seems to reflect a decrease in the number of blue systems, and so we tentatively associate them with satellite dwarf (spheroidal) galaxies that have undergone quenching due to environmental processes.