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

Abstract

We present a new analysis of stellar mass functions in the COSMOS field to fainter limits than has been previously probed at z ≤ 1. The increase in dynamic range reveals features in the shape of the stellar mass function that deviate from a single Schechter function. Neither the total nor the red (passive) or blue (star-forming) galaxy stellar mass functions can be well fitted with a single Schechter function once the mass completeness limit of the sample probes below ~3 × 10^9 M_⊙. We observe a dip or plateau at masses ~10^(10) M_⊙, just below the traditional M^*, and an upturn toward a steep faint-end slope of α ~ –1.7 at lower mass at all redshifts ≤ 1. This bimodal nature of the mass function is not solely a result of the blue/red dichotomy. Indeed, the blue mass function 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 two interpretations for this bimodal distribution. If the gas fraction increases toward lower mass, galaxies with M_ (baryon) ~ 10^(10) M_⊙ 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 ~10^(10) M_⊙. 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 ~10^(11) M_⊙, a phenomenon that naturally arises if the baryon fraction peaks at M_(halo) ~ 10^(12) M_⊙. 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.