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Authors: Julia M. Comerford, Brian F. Gerke, Jeffrey A. Newman, Marc Davis, Renbin Yan, Michael C. Cooper, S.M. Faber, David C. Koo, Alison L. Coil, D.J. Rosario

Date: 19 Oct 2008

Abstract: Because structure in the Universe is built up through galaxy mergers and nearly all galaxies host a central supermassive black hole (SMBH), some galaxies should possess two SMBHs near their centers as the result of a recent merger. These SMBHs spiral to the center of the resultant merger-remnant galaxy, and one or both of the SMBHs may power active galactic nuclei (AGN). Using the DEEP2 Galaxy Redshift Survey we have examined 1881 early-type galaxies, of which 107 exhibit [O III] and Hbeta emission lines indicative of AGN activity. Of these, 37 AGN have [O III] emission-line redshifts significantly different from the redshifts of the host galaxies' stars, corresponding to velocity offsets of ~50 km/s to ~300 km/s. Two of these AGN exhibit double-peaked [O III] emission lines, while the remaining 35 AGN each exhibit a single set of velocity-offset [O III] emission lines. These AGN velocity offsets cannot be readily explained by outflows, but rather are most likely the result of recent galaxy mergers. Based on this interpretation we find that roughly half of early-type galaxies hosting AGN are also merger remnants, which implies that mergers may trigger AGN activity in early-type galaxies. Our result sets a hard lower limit of 2.0% on the fraction of early-type galaxies at redshifts 0.34 < z < 0.82 that have recently undergone mergers. We also find a merger rate of ~3 mergers/Gyr, which includes both minor and major mergers, for early-type galaxies at redshifts 0.34 < z < 0.82.

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