as tr o - ph / 9 40 90 71 27 S ep 9 4 The Contribution of Low Surface - Brightness Galaxiesto Faint Galaxy

Recent observations have revealed a population of blue galaxies at intermediate redshift with a space density well in excess of expectations from the local luminosity function and standard cosmology. The colors and luminosities of these faint blue galaxies are similar to nearby low surface-brightness (LSB) galaxies. Such galaxies are severely underrepresented in surveys used to de ne the local luminosity function, but could in principle be detected in deep surveys. If LSB galaxy density is high enough, the faint-galaxy counts could be explained without requiring rapid galaxy evolution. To explore the consequences of including LSB galaxies, we construct catalogs of simulated non-evolving galaxies drawn from a multivariate distribution of galaxy luminosities, central surface brightnesses, bulge/disk ratios and spectral-energy distributions. We compare two models dominated by LSB galaxies to a \standard" non-evolving model. Model galaxies are convolved with seeing and selected in a manner that closely matches real surveys. For each model we compute the local BJ band luminosity function, HI mass function, number counts in the BJ , I, and K bands, redshift distributions, and color distributions. We nd it possible to include a large population of LSB galaxies and incorporate a steep faint-end slope of the luminosity function in our simulations without violating the constraints on the local eld-galaxy luminosity function or the HI mass function. For q0 = 0:5, the most favorable model matches the counts to B = 23, but falls short of the observations at fainter magnitudes. The discrepancy at faint magnitudes is smaller in the I and K bands. The colors and redshift distributions remain roughly consistent with observations to B = 24. The most serious discrepancy with observations is in the distribution of re at faint magnitudes, suggesting that the model contains too many LSB galaxies. Nevertheless, the results suggest that LSB galaxies could be a signi cant contributor to faint-galaxy counts, reducing the need for such extreme models of galaxy evolution as rapid merging or bursting dwarf galaxies. We propose several tests to assess the contribution of LSB galaxies to faint galaxy counts and to di erentiate models involving moderate galaxy evolution from models involving rapid merging or starbursts.