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

TL;DR

We address the search for recoiling or slingshot massive black holes by identifying spatially offset broad-line regions in MaNGA IFS data. Using a catalog of broad BLR detections and spatial-centroid analyses, the authors compile 14 off-nuclear BLR candidates, of which 6 have SDSS optical counterparts suggesting infalling AGN in mergers, and 8 lack counterparts, remaining viable recoiling/slingshot MBH candidates. Narrow-line diagnostics indicate weaker AGN-ionized NLR emission for the offset BLRs without counterparts, while velocity offsets and BLR luminosities roughly align with recoil predictions, implying a population of MBH ejections in a cosmological context. The inferred surface density of recoiling AGN is compatible with models assuming random spin orientations, and the results underscore the role of mergers in AGN triggering and provide a framework for uniform, population-level constraints on recoiling MBHs. Overall, the study presents a systematic, spectroscopic approach to identify and interpret off-nuclear BLRs as tracers of MBH dynamics and gravitational wave consequences in galaxies.

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.

Figures (12)

• Figure 1: Linear-scale density map showing, for each galaxy, the standard deviations of the broad emission line spaxel FWHMs (normalized by the mean FWHM; $/$) against the standard deviations of the spaxel spatial positions (quadrature sum of the standard deviation of the spaxels in the X- and Y-dimensions) in units of the spatial resolution (/25) for (left) and (right) emission line detections. Only sources with more than one broad emission line spaxel are shown. The distributions of $/$ and /25 (normalized to a sum of unity) are shown along the right and top axes, respectively, for (solid) and (dashed). The concentration of abscissa values at /25 $=$ 0.1 is due to sources with two adjacent broad emission line spaxels. Of the $/$ values, 100% are $<$ 1 for both and detections. Of the values, %, % (, detections) are $<$ 2 $\times$ 25. These distributions show that the line widths and positions of the broad emission line detections in each galaxy are consistent and likely originate from a single source.
• Figure 2: Angular offsets of (left) and (right) BLRs from the host galaxy centroids () against the source extents (; defined in Figure \ref{['fig:SPAXEL_FWHM_STD']}). The dotted gray line denotes the approximate absolute astrometric uncertainty of 01. The dashed black lines denote the threshold of $>$$\times$, and the solid black line denotes the additional uniform offset minimum of $>$ 25 (Section \ref{['sec:offset']}). The parent sample is indicated by the linear-scale density map, while sources that pass the offset criteria are shown as red stars. The concentrations at $=$ 025 and 05 are due to sources with two adjacent spaxels and those with only one spaxel (for which $:=$ 05, i.e., the extent of one spaxel), respectively.
• Figure 3: Best fits to the (continuum-subtracted) spectral regions (in the host galaxy rest-frame) where broad emission lines are detected. For clarity, each spectrum has been normalized by the peak flux. The narrow and broad line components are shown in orange and purple, respectively, and the model sum is shown in red. The uncertainty in the continuum subtracted spectrum is denoted by the gray-shaded region, and the residuals are shown at the bottom. Sorted by right ascension, these are the first two offset BLR sources in the sample. The remainder are shown and discussed in Section \ref{['sec:spectral_models']} of the Appendix (Figure \ref{['fig:spectra_1']}).
• Figure 4: BLR full-width at half-maximum (FWHM; maximum value among all spaxels in each source) against projected physical offset from the host galaxy centroid () for the final sample of spatially offset BLR candidates with (orange squares) and without (magenta circles) optical counterpart detections (Section \ref{['sec:optical']}). The distributions of FWHM and (each normalized to a sum of unity) are shown along the right and top axes, respectively, for the subsets with (orange, dashed) and without (magenta, solid) optical counterparts, and their median values are indicated by straight lines. The distributions of FWHM and are typical of AGN BLRs and late-stage galaxy mergers, respectively.
• Figure 5: Full sample of galaxies with spatially offset BLRs coincident with optical detections from the SDSS imaging. For each galaxy, the panels contain (from left to right) the SDSS $g$$+$$r$$+$$i$ color composite images (with the IFS fiber bundle hexagon shown in magenta), the IFS maps and BPT classifications using the -based criteria (pink $=$ 'AGN', orange $=$ 'composite', and green $=$ 'star forming') and -based criteria (pink $=$ 'Seyfert', purple $=$ 'LINER', and green $=$ 'star forming'), and the and BPT diagrams. In the BPT plots, the solid red lines denote the pure AGN demarcation, while the dashed red lines denote the pure star formation () and LINER () demarcations. The log-scale density map denotes spaxels from the full map (only for spaxels with BPT classifications). The cyan dots denote the subset of the spaxels associated with the BLRs, selected to be within a box (centered on the BLR centroid) of size 2 $\times$ (to include extended narrow line region emission around the BLR) but not including the spaxels that have detected broad emission lines (since the pipeline does not account for them). The BLR positions are indicated by the cyan circles with radii of 25 $+$ (radius used for matching with optical counterparts; Section \ref{['sec:optical']}). The white scale bar indicates 4$"$.