Recoiling Black Hole Candidates from Spatially Offset Broad Emission Lines in MaNGA

Abstract: From the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, we identify 14 off-nuclear broad (FWHM>1000 km/s) Halpha and/or Hbeta emission line sources that indicate spatially offset active galactic nuclei (AGN) candidates. In addition to massive black holes (MBHs) in on-going galaxy mergers, this selection can also find MBHs that have been ejected from the host galaxy nucleus due to MBH binary coalescence and asymmetric gravitational wave emission or the dynamical `slingshot' mechanism. Recoiling/slingshot MBHs are predicted to affect co-evolution between MBHs and their host galaxies, and they are observational tracers of past binary MBH mergers and gravitational wave emission. This is the first systematic search through an integral field spectroscopy survey for ejected MBHs to enable uniform constraints on their surface densities. We find that 42% (6/14) have optical image counterparts consistent with galaxy stellar cores from infalling MBHs before the close binary MBH stage. The remaining 58% (8/14) have large broad line luminosities relative to their stellar core mass upper limits (~2 times larger than for central AGN), suggesting merger-driven MBH accretion enhancements or potentially ejected MBHs. The signatures of AGN-ionized narrow emission lines for recoil/slingshot candidates are weaker by 68%, which is consistent with the ejected MBH scenario. The broad line projected velocity offsets range from ~10-600 km/s and suggest motion within the host galaxy potentials. Finally, the implied recoiling MBH surface density upper limit is consistent with predictions that assume random spin orientations in MBH binaries.