  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 equal-mass dissipationless mergers; rather, they are well
explained by the accretion of disk-dominated galaxies. In particular, the arm-
and looplike 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 the order of 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
disk- and bulge-dominated galaxies in the field, suggest that massive
ellipticals assemble most of their mass well before their tidal debris forms
through the accretion of relatively little, kinematically cold material rather
than in very recent, dissipationless major mergers.
