Faint galaxy counts in a hierarchical universe

Abstract

We interpret galaxy counts in the B and K bands, and the redshift distribution of faint galaxies, using a model for galaxy formation in a hierarchical universe. We first study how variation of each of the free parameters in our model affects its observational predictions. These parameters control processes such as star formation, supernova feedback and merging, and determine the masses and luminosities of the galaxies that populate the evolving distribution of dark matter haloes. We find that more efficient merging results in a galaxy luminosity function with a lower amplitude at the faint end at all epochs. This causes the slope of the faint counts to decrease, and the redshift distribution to shift to higher values of z. We then look at the influence of the parameters that control star formation and feedback in dark matter haloes of differing circular velocity. We find that we can steepen the counts by allowing more stars to form in low-circular-velocity haloes. In order to avoid producing too many low-luminosity galaxies at the present epoch, star formation must stop before z = 0, allowing the galaxy to fade substantially in the B band. The evolution of the satellite galaxies in our models provides a natural way of explaining this phenomenon. Finally, we show that the choice of initial mass function does not influence our results very substantially. We present two models which can fit most of the observational data: (a) a cold dark matter model in which star formaton is suppressed in low-circular-velocity haloes until they are accreted into larger systems, and (b) a mixed dark matter model. Neither model fits perfectly. The amplitude of the field-galaxy luminosity function is too high at the faint end, and we are unable to account for the reddest objects seen at K magnitudes between 16 and 20. Given the uncertainties in both the observational data and the details of our modelling, our results lead us to conclude that neither unphysically high merging rates nor new populations of objects are needed to explain the observations in the standard framework of hierarchical clustering in an Ω0 = 1 universe.