Inspiralling supermassive black holes: A new signpost for galaxy mergers

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, Aaron A. Dutton

Research output: Contribution to journalArticlepeer-review

163 Scopus citations


We present a new technique for observationally identifying galaxy mergers spectroscopically rather than through host galaxy imaging. Our technique exploits the dynamics of supermassive black holes (SMBHs) powering active galactic nuclei (AGNs) in merger-remnant galaxies. Because structure in the universe is built up through galaxy mergers and nearly all galaxies host a central 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 AGNs. Using the DEEP2 Galaxy Redshift Survey, we have examined 1881 red galaxies, of which 91 exhibit [O III] and Hβ emission lines indicative of Seyfert 2 activity. Of these, 32 AGNs have [O III] emission-line redshifts significantly different from the redshifts of the host galaxies' stars, corresponding to velocity offsets of 50 km s-1 to 300 km s-1. Two of these AGNs exhibit double-peaked [O III] emission lines, while the remaining 30 AGNs each exhibit a single set of velocity-offset [O III] emission lines. After exploring a variety of physical models for these velocity offsets, we argue that the most likely explanation is inspiralling SMBHs in merger-remnant galaxies. Based on this interpretation, we find that roughly half of the red galaxies hosting AGNs are also merger remnants, which implies that mergers may trigger AGN activity in red galaxies. The AGN velocity offsets we find imply a merger fraction of 30% and a merger rate of 3 mergers Gyr-1 for red galaxies at redshifts 0.34 < z < 0.82.

Original languageEnglish
Pages (from-to)956-965
Number of pages10
JournalAstrophysical Journal
Issue number1
StatePublished - 2009


  • Galaxies: Seyfert
  • Galaxies: active
  • Galaxies: interactions
  • Galaxies: nuclei

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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