Tidal debris in elliptical galaxies as tracers of mergers with disks

We use a set of high-resolution N-body simulations of binary galaxy mergers to show that the morphologies of the tidal features that are seen around a large fraction of nearby, massive ellipticals in the field, cannot be reproduced by equalmass dissipationless mergers; rather, they are well explained by the accretion of disk-dominated galaxies. In particular, the armand loop-like morphologies of the observed tidal debris can only be produced by the kinematically-cold material of the disk components of the accreted galaxies. The tidal features that arise from such ”cold-accretion” events onto a massive elliptical are visible for significantly longer timescales than the features produced by elliptical-elliptical mergers (about 1-2 Gyr vs. a few hundred million years). Mass ratios of order 1 : 10 between the accreting elliptical and the accreted disk are sufficient to match the brightness of the observed debris. Furthermore, stellar population synthesis models and simple order of magnitude calculations indicate that the colors of the tidal features generated in such minor cold-accretion events are relatively red, in agreement with the observations. The minor cold-accretion events that explain the presence, brightness and structural and color properties of the tidal debris, cause only a modest mass and luminosity increase in the accreting massive elliptical. These results, coupled with the relative statistical frequencies of diskand bulge-dominated galaxies in the field, imply that the most massive ellipticals assemble well before their tidal debris forms through the accretion of relatively little, kinematically-cold material, rather than in very recent, dissipationless major mergers. Subject headings: galaxies: interactions — galaxies: elliptical and lenticular, cD — galaxies: spiral Department of Physics, Institute of Astronomy, ETH Zürich, Wolfang-Pauli-Strasse 16, 8093 Zürich, Switzerland Department of Physics, Institute of Theoretical Physics, University of Zurich, Winterthurerstrasse 190, 8057 Zürich, Switzerland ar X iv :0 80 1. 47 64 v1 [ as tr oph ] 3 0 Ja n 20 08