On the epoch of elliptical galaxy formation

This paper addresses the question of how recently the onset of star formation occurred in systems which evolved to become present-day elliptical galaxies. The emphasis here is on the implications of varying the formation redshift for ellipticals, keeping the modeled star formation history invariant. The approach taken is to calculate observable quantities from a deliberately simple model for the photometric evolution of a stellar population, formed in a burst of one billion-year duration, with a Salpeter initial mass function, and of solar metallicity. The photometric evolution is calculated on the basis of Renzini's (1981) results, which include contributions from post-red giant stellar evolutionary phases. The redshift of formation and cosmological parameters assumed in the model calculations are varied to allow investigation of a range of look-back times to the initial burst of star formation. Comparison with a range of observed properties of high-redshift galaxies marginally favor cosmological parameters which maximize the age corresponding to given redshift, i.e., a low-density universe or a low Hubble constant or both. The present observational constraints are consistent with galaxy formation occurring as recently as redshifts less than 3.