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 disk merger scenario based on the stellar populations of disk galaxies. We draw the following conclusions: low-redshift collisionless or gaseous mergers of present-day Milky-Way-like disk galaxies do not form present-day elliptical galaxies. Binary mergers of the progenitors of present-day Milky-Way-like disk galaxies could have evolved into intermediate-mass elliptical galaxies (M < M *) if they merged earlier than ≈ 3-4 Gyr ago. Assuming that most present-day disk galaxies formed in a way similar to the Milky Way model presented here, more massive giant ellipticals in general could not have formed from binary mergers of the progenitors of present-day disk galaxies. A major reason for these conclusions is that the mass in metals of a 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.