The Fate of Merger Remnants

Using published luminosity and molecular gas profiles of the merger remnants NGC 3921, NGC 7252 and Arp 220, we examine the expected morphological and kinematic structure of the resulting merger remnants, especially in light of the massive CO complexes that are observed in their nuclei. For NGC 3921 and NGC 7252 we find that even under the most optimistic estimates (normal IMF, no mass ejection, 100% efficiency of turning gas into stars, fading the present population by 1 mag), the resulting luminosity profile should still be characterized by an r law. In view of previous optical work on these systems, it seems likely that they will evolve into normal ellipticals as regards their optical properties. Due to a much higher central molecular column density, Arp 220 might not evolve such a “seamless” light profile. We conclude that ultraluminous infrared mergers such as Arp 220 either evolve into ellipticals with luminosity spikes, or do not produce many low-mass stars out of their molecular gas complexes. Finally, we examine the differences between the molecular gas morphology and kinematics of NGC 3921 and NGC 7252, suggesting that whether a remnant is boxy or disky may depend on the gas content of the progenitors. Subject headings: galaxies: individual (NGC 3921, NGC 7252, Arp 220) — galaxies: interactions — galaxies: evolution — ISM: molecules — infrared: galaxies