Are disk galaxies the progenitors of giant ellipticals?

Abstract

A popular formation scenario for giant elliptical galaxies proposes that they might have formed from binary mergers of disk galaxies. Difficulties with the scenario that emerged from earlier studies included providing the necessary stellar mass and metallicity, maintaining the tight color-magnitude relation and avoiding phase space limits. In this paper we revisit the issue and put constraints on the binary disc merger scenario based on the stellar populations of disc galaxies. We draw the following conclusions: Low redshift collisionless or gaseous mergers of present day Milky Way like disc galaxies do not form present day elliptical galaxies. Binary mergers of the progenitors of present day Milky Way like disc galaxies can have evolved into intermediate mass elliptical galaxies ($M < M_*$) if they have merged earlier than $\approx$ 3-4 Gyrs ago. Assuming that most present day disk galaxies formed in a similar way to the Milky Way model presented here, more massive giant ellipticals in general can not have formed from binary mergers of the progenitors of present day disc galaxies. A major reason for these conclusions is that the mass in metals of typical disk galaxy is approximately a factor of 4-8 smaller than the mass in metals of a typical early-type galaxy and this ratio grows to larger values with increasing redshift.Comment: accepted by ApJ, to appear in Jan 200