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We present a new analysis of stellar mass functions (MF) in the COSMOS fieldto fainter limits than has been previously probed to z~1. Neither the total northe passive or star-forming MF can be well fit with a single Schechter functiononce one probes below 3e9 Msun. We observe a dip or plateau at masses ~1e10Msun, and an upturn towards a steep faint-end slope of -1.7 at lower mass atany z<1. This bimodal nature of the MF is not solely a result of the blue/reddichotomy. The blue MF is by itself bimodal at z~1. This suggests a newdichotomy in galaxy formation that predates the appearance of the red sequence.We propose two interpretations for this bimodality. If the gas fractionincreases towards lower mass, galaxies with M_baryon~1e10 Msun would shift tolower stellar masses, creating the observed dip. This would indicate a changein star formation efficiency, perhaps linked to supernovae feedback becomingmuch more efficient. Therefore, we investigate whether the dip is present inthe baryonic (stars+gas) MF. Alternatively, the dip could be created by anenhancement of the galaxy assembly rate at ~1e11 Msun, a phenomenon thatnaturally arises if the baryon fraction peaks at M_halo ~1e12 Msun. In thisscenario, galaxies occupying the bump around M* would be identified withcentral galaxies and the second fainter component having a steep faint-endslope with satellites. While the dip is apparent in the total MF at any z, itappears to shift from the blue to red population, likely as a result oftransforming high-mass blue galaxies into red ones. At the same time, we detecta drastic upturn in the number of low-mass red galaxies. Their increase withtime reflects a decrease in the number of blue systems and so we tentativelyassociate them with satellite dwarf galaxies that have undergone quenching.

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