NOEMA Detection of Circumnuclear Molecular Gas in X-ray Weak Dual Active Galactic Nuclei: No Evidence for Heavy Obscuration

TL;DR

This study uses NOEMA observations to detect molecular gas in dual AGN systems, finding no evidence of heavy obscuration and suggesting AGN feedback may suppress further accretion during galaxy mergers.

Contribution

First detailed molecular gas study of low-redshift dual AGN pairs showing low X-ray luminosity, revealing moderate obscuration and feedback effects during galaxy mergers.

Findings

Most nuclei show compact, rotating molecular gas structures.

Column densities are moderate, not heavily obscured.

Feedback likely suppresses SMBH accretion post-merger.

Abstract

Dual active galactic nuclei (AGN), which are the manifestation of two actively accreting supermassive black holes (SMBHs) hosted by a pair of merging galaxies, are a unique laboratory for studying the physics of SMBH feeding and feedback during an indispensable stage of galaxy evolution. In this work, we present NOEMA CO(2-1) observations of seven kpc-scale dual-AGN candidates drawn from a recent Chandra survey of low-redshift, optically classified AGN pairs. These systems are selected because they show unexpectedly low 2-10 keV X-ray luminosities for their small physical separations signifying an intermediate-to-late stage of merger. Circumnuclear molecular gas traced by the CO(2-1) emission is significantly detected in 6 of the 7 pairs and 10 of the 14 nuclei, with an estimated mass ranging between $(0.2 - 21) \times10^9\rm~M_{\odot}$. The primary nuclei, i.e., the ones with the higher stellar velocity dispersion, tend to have a higher molecular gas mass than the secondary. Most CO-detected nuclei show a compact morphology, with a velocity field consistent with a kpc-scale rotating structure. The inferred hydrogen column densities range between $5\times10^{21} - 2\times10^{23}\rm~cm^{-2}$, but mostly at a few times $10^{22}\rm~cm^{-2}$, in broad agreement with those derived from X-ray spectral analysis. Together with the relatively weak mid-infrared emission, the moderate column density argues against the prevalence of heavily obscured, intrinsically luminous AGNs in these seven systems, but favors a feedback scenario in which AGN activity triggered by a recent pericentric passage of the galaxy pair can expel circumnuclear gas and suppress further SMBH accretion.