Pre-enriched, not primordial ellipticals

Abstract

We follow the chemical evolution of a galaxy through star formation and its feedback into the inter-stellar medium (ISM), starting from primordial gas and allowing for gas to inflow into the region being modelled. We attempt to reproduce observed spectral line-strenghts for early-type galaxies to constrain their star formation histories (SFH). The efficiencies and times of star formation are varied as wellas the amount and duration of inflow. We evaluate the chemical enrichment and the mass of stars made with time. Single stellar population (SSP) data are then used to predict line-strengths for composite stellar populations. The results are compared with observed line-strengths in ten ellipticals, including some features which help to break the problem of agemetallicity degeneracy in old stellar populations. We find that the elliptical galaxies modelled require high metallicity SSPs (> 3 Z⊙) at later times. In addition the strong lines observed cannot be produced by an initial starburst in primordial gas, even if a large amount of inflow is allowed for during the first few ×10 8 years. This is because some pre-enrichment is required for lines in the bulk of the stars to approach the observed line-strengths in ellipticals. These strong lines are better modelled by a system with a delayed burst of star formation, following an early SFH which can be a burst or more steady star formation. Such a model is representative of star formation in normal ellipticals or spirals respectively, followed by a starburst and gas inflow during a merger or strong interaction with a gas-rich galaxy. Alternatively, a single initial burst of normal stars with a Salpeter initial mass function could produce the observed strong lines if it followed some pre-enrichment process which did not form long-lived stars (e.g. population III stars).