The Bimodal Galaxy Stellar Mass Function in the COSMOS Survey to z ~ 1: A Steep Faint End and a New Galaxy Dichotomy

International audienceWe present a new analysis of stellar mass functions in the COSMOS field to fainter limits than has been previously probed at z 9 M sun. We observe a dip or plateau at masses ~1010 M sun, just below the traditional M*, and an upturn toward a steep faint-end slope of α ~ -1.7 at lower mass at all redshifts baryon ~ 1010 M sun would shift to lower stellar masses, creating the observed dip. This would indicate a change in star formation efficiency, perhaps linked to supernovae feedback becoming much more efficient below ~1010 M sun. Therefore, we investigate whether the dip is present in the baryonic (stars+gas) mass function. Alternatively, the dip could be created by an enhancement of the galaxy assembly rate at ~1011 M sun, a phenomenon that naturally arises if the baryon fraction peaks at M halo ~ 1012 M sun. In this scenario, galaxies occupying the bump around M * would be identified with central galaxies and the second fainter component of the mass function having a steep faint-end slope with satellite galaxies. The low-mass end of the blue and total mass functions exhibit a steeper slope than has been detected in previous work that may increasingly approach the halo mass function value of -2. While the dip feature is apparent in the total mass function at all redshifts, it appears to shift from the blue to the red population, likely as a result of transforming high-mass blue galaxies into red ones. 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