Molecular Gas in Major Mergers Hosting Dual and Single AGN at <10 kpc Nuclear Separations

TL;DR

This work uses high-resolution ALMA CO data to dissect molecular gas in seven local major mergers hosting dual and single AGN. By applying KinMS-based Bayesian MCMC rotating-disk models, the authors separate rotating gas from a non-rotating component within SMBH spheres of influence and link gas properties to SMBH mass and activity. They find that more massive SMBHs have higher surface densities of non-rotating gas in their SoIs, yet this gas does not correlate with current accretion efficiency, implying only part of the gas feeds the SMBH. The study also finds no strong molecular-gas differences between single and dual AGN hosts, suggesting that dual AGN occurrence is more influenced by AGN variability and obscuration than by available nuclear gas. Overall, the results highlight how merger-driven gas inflows reshape nuclear gas distributions and inform SMBH growth pathways in the most gas-rich galactic environments.

Abstract

We present high-resolution ($\sim$50$-$100 pc) Atacama Large Millimeter Array (ALMA) observations of $^{12}$CO(2-1) or $^{12}$CO(1-0) emission in seven local ($z$ $\lesssim$ 0.05) major mergers -- five of which are dual active galactic nuclei (AGN) systems, and two of which are single AGN systems. We model the molecular gas kinematics through rotating disk profiles using a Bayesian Markov chain Monte Carlo approach. The residuals were then used to isolate non-rotating components of the molecular gas -- the most likely contributor to future SMBH growth. We find that more massive SMBHs have higher surface densities of non-rotating molecular gas within their sphere of influence. This potential molecular gas supply, however, does not correlate with the current accretion efficiency of the SMBHs, suggesting that only a fraction of the observed non-rotating gas is currently reaching the SMBH. Finally, we tentatively find no significant differences in the nuclear molecular gas masses of single AGN and dual AGN hosts, both within the SMBH sphere of influence and within the central kiloparsec. Our results indicate that the probability of occurrence of the dual AGN phenomenon is likely dependent on AGN variability and/or obscuration rather than the availability of molecular gas in the nuclear regions.

Figures (23)

• Figure 1: High-resolution $^{12}$CO moment 0 (integrated intensity) maps of confirmed dual AGN in major mergers. When resolved, upper-bound and lower-bound estimates of SMBH SoI are overlaid as cyan and yellow circles, respectively. If only the SoI upper-bound is resolved, we only show the upper-bound. For systems with unresolved SoIs, we mark the AGN position with a white star. Here, we analyze $^{12}$CO(1-0) for ESO 253-G003 emission, whereas the remaining systems only have $^{12}$CO(2-1) detections. Note that the colormap is shown on a logarithmic scale.
• Figure 2: High-resolution $^{12}$CO(2-1) moment 0 (integrated intensity) maps of single AGN in major mergers. For the AGN, the upper-bound and lower-bound estimates of the SMBH SoI are overlaid as cyan and yellow circles, respectively. The white star indicates an unresolved SoI. For the non-AGN SMBH, we mark the upper-bound of the SoI with a magenta circle (lower-bounds were unresolved). Note that the colormap is shown on a logarithmic scale.
• Figure 3: $^{12}$CO(2-1) Moment 8 (spectral line peak) map of Mrk 739. In addition to the prominent $^{12}$CO emission from Mrk 739W, the circumnuclear ring of Mrk 739E is faintly visible to the left-hand side. The SMBHs have unresolved SoIs, and we mark their positions with white stars. Note that the colormap is shown on a logarithmic scale.
• Figure 4: Best-fit models created using KinMS for dual AGN systems. Each row of panels refers to a single system. The left-hand column of panels refers to the zeroth moment (integrated intensity) of the $^{12}$CO data cube, whereas the right-hand column refers to the first moment (velocity profile). From left to right, each column shows the data, model, and residual map. Primarily for visual purposes, the data, model, and residual maps have been masked to spatial regions that had pixels with $\geq3\sigma$$^{12}$CO detections in the ALMA cube. For Mrk 739E and ESO 253-G003 SW, however, we lower this threshold to $\geq2\sigma$ to account for the faintness of the emission. The best-fit position angle of the rotating disk is shown as a dashed black line. For systems with resolved SoIs, we show the lower and upper bounds of the SoIs as black circles. If only the SoI upper bound is resolved, we only show the upper bound. For systems with unresolved SoIs at both bounds, we mark the AGN position as a star.
• Figure 5: cont.