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Published April 1, 2011 | Published
Journal Article Open

The dirt on dry mergers


Using data from the Spitzer Space Telescope, we analyze the mid-infrared (3-70 μm) spectral energy distributions of dry merger candidates in the Boötes 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 μm 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 M_⊙ yr^(–1). 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 star-forming 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.

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© 2011 American Astronomical Society. Received 2010 December 3; accepted 2011 February 1; published 2011 March 14. A.D. thanks the SSC/Caltech for its gracious hospitality during summer 2009, when much of this paper was written. E.C. was supported through the Caltech Summer Undergraduate Research Fellowship program and the Spitzer Enhanced Science Fund. E.L.F. is supported by the Spitzer Fellowship Program through a contract with JPL/Caltech/NASA. We thank John Moustakas for advice on using the SDSS spectra, and Stéphane Charlot for providing the CB07 models. We also warmly thank Lee Armus, George Helou, Bradford Holden, Daniel Kelson, Francine Marleau, Samir Salim, and Nick Scoville for stimulating discussions pertaining to this work. Finally, we are grateful to the anonymous referee for providing useful feedback that improved this work. This research is partially supported by the National Optical Astronomy Observatory that is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the National Science Foundation. This work is in part based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. The Spitzer/MIPS survey of the Boötes region was obtained using GTO time provided by the Spitzer Infrared Spectrograph Team (James Houck, P. I.) and by M. Rieke. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U. S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck- Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington. Facilities: Spitzer (MIPS,IRAC,IRS), KPNO:2.1m (ONIS, SQUID,FLAMINGOS,FLAMINGOS-1), Mayall (Mosaic-1)

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