Probing the evolution of early-type cluster galaxies through chemical enrichment

We describe a simple chemical enrichment model for cluster early-type galaxies, in which the main mechanisms considered in the evolutionary model are infall of primordial gas, outflows, and a possible variation in the star formation efficiency. We find that-within the framework of our models-only outflows can generate the suitable range of metallicities needed in order to explain the color-magnitude relation. The chemical enrichment tracks can be combined with the latest population synthesis models from Bruzual & Charlot to simulate clusters over a wide redshift range, for a set of toy models with different infall rates, star formation efficiencies, and star formation scenarios. The color-magnitude relation of local clusters is used as a constraint, fixing the correlation between absolute luminosity and the ejected fraction of gas from outflows. We find that the correlations between color or mass-to-light ratios and absolute luminosity are degenerate with respect to most of the input parameters. However, a significant change between monolithic and hierarchical models is predicted for redshifts z greater than or similar to 1. The most important observable that differentiates between these alternative formation scenarios is the population of blue early-type galaxies that fall conspicuously blueward of the red envelope. The comparison between predicted and observed mass-to-light ratios yields an approximate linear bias between total and stellar masses, M-tot proportional to M-st(1.15+/-0.08), in early-type galaxies. If we assume that outflows constitute the driving mechanism for the colors observed in cluster early-type galaxies, the metallicity of the intracluster medium (ICM) can be linked to outflows; the color-magnitude constraint requires faint (M-V similar to -16) galaxies to eject 85% of their gas, which means that most of the metals in the ICM may have originated in these dwarf galaxies. No significant evolution is predicted, in agreement with X-ray observations by Mushotzky & Loewenstein. Other mechanisms contributing to the enrichment of the ICM, such as ejected material from mergers that formed the largest ellipticals, should translate into a decrease of the intracluster metallicity at z greater than or similar to 1-1.5. Forthcoming observations from Chandra and XMM will help elucidate this point.