The Evolving Faint End of the Luminosity Function

We investigate the evolution of the faint-end slope of the luminosity function, α, using semianalytical modeling of galaxy formation. In agreement with observations, we find that the slope can be fitted well by α(z) = a + bz, with a = -1.13 and b = -0.1. The main driver for the evolution in α is the evolution in the underlying dark matter mass function. Sub-L* galaxies reside in dark matter halos that occupy a different part of the mass function. This part of the mass function is steeper at high redshifts than at low redshifts, and hence α is steeper. Supernova feedback in general causes the same relative flattening with respect to the dark matter mass function. The faint-end slope at low redshifts is dominated by field galaxies, and at high redshifts by cluster galaxies. The evolution of α(z) in each of these environments is different, with field galaxies having a slope b = -0.14 and cluster galaxies having a slope b = -0.05. The transition from a cluster-dominated to a field-dominated faint-end slope occurs roughly at a redshift z* 2 and suggests that a single linear fit to the overall evolution of α(z) might not be appropriate. Furthermore, this result indicates that tidal disruption of dwarf galaxies in clusters cannot play a significant role in explaining the evolution of α(z) at z < z*. In addition, we find that different star formation efficiencies a* in the Schmidt-Kennicutt law and supernova-feedback efficiencies generally do not strongly influence the evolution of α(z).