A Predicted New Population of UV-faint Galaxies at z>4

We show that a bursty model of high redshift star formation explains several puzzling observations of the high redshift galaxy population. We begin by pointing out that the observed specific star formation rate requires a duty-cycle of ~10%, which is much lower than found in many hydro-dynamical simulations. This value follows directly from the fact that the observed star formation rate in galaxies integrated over a Hubble time exceeds the observed stellar mass by an order of magnitude. We use the large observed specific star formation rate to calibrate the efficiency of feedback in a model for the high redshift star formation rate which includes merger driven star formation regulated by SNe feedback. This model reproduces the star formation rate density function and the stellar mass function of galaxies at 4<z<7. A prediction of the model is that the specific star formation rate does not evolve very rapidly with either mass or redshift, in agreement with observation. This is in contrast to results from hydrodynamical simulations where the star formation closely follows the accretion rate, and so increases strongly towards high redshift. The bursty star formation model naturally explains the observation that there is not enough stellar mass at z~2-4 to account for all of the star-formation observed, without invoking properties like an evolving initial mass function of stars. The finding of a duty cycle that is ~10% implies that there should be ten times the number of known galaxies at fixed stellar mass that have not yet been detected through standard UV selection at high redshift. We therefore predict the existence of a large undetected population of UV-faint galaxies that accounts for most of the stellar mass density at z~4-8.