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.