Probing Star Formation Timescales in Elliptical Galaxies

In models of galaxy formation in a hierarchical Universe, elliptical galaxies form through the merging of smaller disk systems. These models yield a number of testable predictions if reliable techniques for determining the relative ages and compositions of the stellar populations of different galaxies can be found: 1) ellipticals in low-density environments form later than ellipticals in clusters, 2) more massive ellipticals form later, 3) more massive ellipticals form in dissipationless mergers from disk galaxies with low gas content. While colours and the Balmer line strengths of galaxies can be used to infer the average ages of the stellar populations of ellipticals, α/Fe element ratios carry information about the timescale over which star formation took place. Here we present preliminary results from semi-analytic models for the distribution of Mg/Fe ratios in galaxies as a function of morphological type, luminosity and environment. 1. Hierarchical Clustering and the Formation of Ellipticals through Mergers According to the standard theoretical paradigm, the structures observed in the Universe today were formed by the gravitational amplification of small perturbations in an initially Gaussian dark matter density field. Small scale overdensities were the first to collapse, and the resulting objects subsequently merged under the influence of gravity to form larger structures such as groups and clusters of galaxies. Galaxies formed within dense halos of dark matter, where gas was able to reach high enough overdensities to cool, condense and form stars. The quiescent cooling of gas within a dark matter halo results in the formation of a rotationally-supported disk system at the centre of the halo. When halos merge with each other, a bound group of galaxies is produced. Dynamical friction will cause the orbits of the group members to erode over time, and the galaxies to spiral in towards the centre of the halo and merge. Galaxygalaxy mergers are thus inevitable in this picture. N-body simulations have demonstrated that mergers between disk galaxies of near-equal mass result in the formation of remnant systems that are structurally very similar to observed elliptical galaxies (e.g. Barnes & Hernquist 1996).