Faint Blue Galaxies and the Epoch of Dwarf-Galaxy Formation

Abstract

Both theoretical and observational lines of reasoning suggest that the very faint (B > 24) galaxies seen in deep images of the sky are small low-mass galaxies that experienced starburst at 0.5<z<1 and have since faded into low luminosity, low surface brightness objects. We scrutinize this hypothesis in order to determine whether such a model can account for the observed wavelength- dependent number counts as well as redshift, color and size distributions. Low-mass galaxies generically arise in large numbers in hierarchical clustering scenarios. Although expected to form at high redshifts, the metagalactic UV radiation at high redshifts will delay significant star formation (SF) activity in these galaxies until z < 1. We combine these two elements, along with simple assumptions about SF histories and efficiency, to construct our bursting dwarf model. We further augment the model with a phenomenological prescription for the formation and evolution of the locally observed galaxies, and use spectral synthesis/Monte-Carlo methods to generate realistic galaxy catalogs for comparison with observations. For reasonable choices of SF histories for the low-mass galaxies, the model results are in very good agreement with the results of the deep galaxy surveys. Moreover, the results of deep HST imaging studies, of gravitational lensing and galaxy clustering studies, and of regular galaxies at intermediate/high redshifts favor the dwarf-dominated model. We also discuss some generic predictions of the model. For example, the model predicts that the K-band number counts should begin to rise more steeply at magnitudes fainter than K_{AB} = 24-25 and the local field luminosity function should steepen at magnitudes fainter than M_B \approx -16. We also consider the possibility of detecting the low surface