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Published September 20, 2010 | Published
Journal Article Open

A Multiwavelength Study of a Sample of 70 μm Selected Galaxies in the COSMOS Field. II. The Role of Mergers in Galaxy Evolution


We analyze the morphological properties of a large sample of 1503 70 μm selected galaxies in the COSMOS field spanning the redshift range 0.01 < z < 3.5 with a median redshift of 0.5 and an infrared luminosity range of 10^8 < L_(IR)(8 – 1000 μm)< 10^(14) L_☉ with a median luminosity of 10^(11.4) L_☉. In general, these galaxies are massive, with a stellar mass range of 10^(10)-10^(12) M_☉, and luminous, with –25 < M_K < –20. We find a strong correlation between the fraction of major mergers and L IR, with the fraction at the highest luminosity (L_(IR) > 10^(12) L_☉) being up to ~50%. We also find that the fraction of spirals drops dramatically with L_(IR). Minor mergers likely play a role in boosting the infrared luminosity for sources with low luminosities (L_(IR) < 10^(11.5) L_☉). The precise fraction of mergers in any given L(IR) bin varies by redshift due to sources at z > 1 being difficult to classify and subject to the effects of bandpass shifting; therefore, these numbers can only be considered lower limits. At z < 1, where the morphological classifications are most robust, major mergers clearly dominate the ULIRG population (~50%-80%) and are important for the LIRG population (~25%-40%). At z > 1, the fraction of major mergers is lower, but is at least 30%-40% for ULIRGs. In a comparison of our visual classifications with several automated classification techniques we find general agreement; however, the fraction of identified mergers is underestimated due to automated classification methods being sensitive to only certain timescales of a major merger. Although the general morphological trends agree with what has been observed for local (U)LIRGs, the fraction of major mergers is slightly lower than seen locally. This is in part due to the difficulty of identifying merger signatures at high redshift. The distribution of the U – V color of the galaxies in our sample peaks in the green valley (= 1.1) with a large spread at bluer and redder colors and with the major mergers peaking more strongly in the green valley than the rest of the morphological classes. We argue that, given the number of major gas-rich mergers observed and the relatively short timescale that they would be observable in the (U)LIRG phase, it is plausible for the observed red sequence of massive ellipticals (<10^(12) M_☉) to have been formed entirely by gas-rich major mergers.

Additional Information

© 2010 American Astronomical Society. Received 2009 August 9; accepted 2010 June 22; published 2010 August 26. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc, under NASA contract NAS 5-26555; also based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; the European Southern Observatory under Large Program 175.A-0839, Chile; the National Radio Astronomy Observatory which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc; the Canada–France–Hawaii Telescope with MegaPrime/MegaCam operated as a joint project by the CFHT Corporation, CEA/DAPNIA, the National Research Council of Canada, the Canadian Astronomy Data Centre, the Centre National de la Recherche Scientifique de France, TERAPIX and the University of Hawaii. Support for this work was provided in part by NASA through contracts 1282612, 1298213, and 1344920 issued by the Jet Propulsion Laboratory. This work was supported in part by a grant from The City University of New York PSCCUNY Research Award Program. We would also like to recognize the contributions from all of the members of the COSMOS Team who helped in obtaining and reducing the large amount of multi-wavelength data that are now publicly available through the NASA Infrared Science Archive (IRSA) at http://irsa.ipac.caltech.edu/Missions/cosmos.html. The analysis pipeline used to reduce the DEIMOS data was developed at UC Berkeley with support from NSF grant AST-0071048. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has also made use of data from the Sloan Digital Sky Survey (SDSS-DR7). Funding for the SDSS and SDSSII 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 SDSSWeb site is http://www.sdss.org/. We thank the anonymous referee for their thorough comments, which greatly improved the paper. We also thank Marco Barden and Knud Jhanke for the use of the FERENGI redshifting code as well as their help with using it.

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