The Dirt on Dry Mergers
  Using data from the Spitzer Space Telescope, we analyze the mid-infrared
(3-70 micron) spectral energy distributions of dry merger candidates in the
Bootes field of the NOAO Deep Wide-Field Survey. These candidates were selected
by previous authors to be luminous, red, early-type galaxies with morphological
evidence of recent tidal interactions. We find that a significant fraction of
these candidates exhibit 8 and 24 micron excesses compared to expectations for
old stellar populations. We estimate that a quarter of dry merger candidates
have mid-infrared-derived star formation rates greater than ~1 MSun/yr. This
represents a "frosting" on top of a large old stellar population, and has been
seen in previous studies of elliptical galaxies. Further, the dry merger
candidates include a higher fraction of starforming galaxies relative to a
control sample without tidal features. We therefore conclude that the star
formation in these massive ellipticals is likely triggered by merger activity.
Our data suggest that the mergers responsible for the observed tidal features
were not completely dry, and may be minor mergers involving a gas-rich dwarf
galaxy.
