Constraints on galaxy formation from α-enhancement in luminous elliptical galaxies

Abstract

We explore the formation of α-enhanced and metal-rich stellar populations in the nuclei of luminous ellipticals under the assumption of two extreme galaxy formation scenarios based on hierarchical clustering, namely a fast clumpy collapse and the merger of two spirals. We investigate the parameter space of the star formation time-scale, the slope of the initial mass function (IMF) and stellar yields. In particular, the latter add a huge uncertainty in constraining time-scales and IMF slopes. We find that - for Thielemann, Nomoto & Hashimoto nucleosynthesis - in a fast clumpy collapse scenario an [α/Fe] overabundance of ∼0.2 dex in the high-metallicity stars can be achieved with a Salpeter IMF and star formation time-scales of the order 109 yr. The scenario of two merging spirals that are similar to our Galaxy, instead, fails to reproduce the α-enhanced abundance ratios in the metal-rich stars, unless the IMF is flattened during the burst ignited by the merger. This result is independent of the burst time-scale. We suggest that abundance gradients give hints to distinguish between the two extreme formation scenarios considered in this paper.