FAINT BLUE GALAXIES AND THE EPOCH OF DWARF GALAXY FORMATION

Several independent lines of reasoning, both theoretical and observati onal, suggest that the very faint (B greater than or equal to 24) gala xies seen in deep images of the sky are small low-mass galaxies that e xperienced a short starburst at redshifts 0.5 less than or equal to z less than or equal to 1 and have since faded into low-luminosity, low surface brightness (LSB) objects. We examine this hypothesis in detail in order to determine whether a model incorporating such dwarfs can a ccount for the observed wavelength-dependent number counts, as well as redshift, color, and size distributions. Low-mass galaxies genericall y arise in large numbers in hierarchical clustering scenarios with rea listic initial conditions. Generally, these galaxies are expected to f orm at high redshifts. Babul & Rees have argued that the formation epo ch of these galaxies is, in fact, delayed until z less than or equal t o 1 due to the photoionization of the gas by the metagalactic UV radia tion at high redshifts. We combine these two elements, along with simp le heuristic assumptions regarding star formation histories and effici ency, to construct our bursting dwarf model. The slope and the normali zation of the mass function of the dwarf galaxies are derived from the initial conditions and are not adjusted to fit the data. We further a ugment the model with a phenomenological prescription for the formatio n and evolution of the locally observed population of galaxies (E, SO, Sab, Sbc, and Sdm types). We use spectral synthesis and Monte Carlo m ethods to generate realistic model galaxy catalogs for comparison with observations. We find that for reasonable choices of the star formati on histories for the dwarf galaxies, the model results are in very goo d agreement with the results of the deep galaxy surveys. Such a dwarf- dominated model is also qualitatively supported by recent studies of f aint galaxy gravitational lensing and clustering, by galaxy size distr ibutions measured with the Hubble Space Telescope, and by the evidence for very modest evolution in regular galaxy properties out to z = 1. We also discuss various tests of the model based on some generic predi ctions. For example, the model predicts that the number counts in the K band ought to begin rising more steeply at magnitudes fainter than K -AB approximate to 24-25. The model also predicts that the local field luminosity function (LF) ought to exhibit a steep upturn at magnitude s fainter than M(B) approximate to -16. The detection of the latter, h owever, depends sensitively on the selection criteria used to construc t a galaxy catalog. We also consider the possibility of detecting the LSB remnants at low redshifts.