Probing Heavily Obscured AGN in Major Galaxy Mergers Using the mm-X-ray Correlation

TL;DR

This study addresses the challenge of census gaps for heavily obscured SMBH growth in late-stage mergers by testing the millimeter–X-ray luminosity correlation as a diagnostic tool. Using ALMA Band 6 data and hard X-ray measurements (2–10 keV and 14–150 keV) from GOALS (ULIRG/LIRG) systems, the authors quantify mm and X-ray emissions, account for star-formation contamination, and extrapolate X-ray luminosities when direct measurements are unavailable. They find that several systems, including confirmed dual AGN, lie near the mm–X-ray relation within ~3σ, and some sources require a combination of star formation and obscured AGN to explain their luminosities; spatially extended mm emission is often dominated by star formation rather than nuclear AGN activity at ACA resolutions. The results suggest that mm continuum emission offers a valuable, dust-insensitive tracer for obscured SMBH growth and can aid in identifying hidden AGN pairs in mergers, with ALMA enabling robust tests and future facilities like the ngVLA expanding applicability to larger and more distant samples.

Abstract

The study of heavily obscured supermassive black hole (SMBH) growth in late-stage galaxy mergers is challenging: column densities $N_{\mathrm{H}}>10^{24},\mathrm{cm}^{-2}$ can block most nuclear emission, leaving significant gaps in the SMBH growth census. Millimeter-wave continuum emission offers a potential window into this obscured phase, as it can trace Active Galactic Nuclei (AGN) activity through mechanisms less affected by dust extinction. In this work, we test whether the observed correlation between millimeter ($\sim200,\mathrm{GHz}$) and hard X-ray (14 - 150,keV) luminosities can be used to plausibly identify hidden AGN in local (Ultra)Luminous Infrared Galaxies (U)LIRGs, including systems hosting confirmed dual AGN. We identify three sources -- one confirmed AGN and two strong candidates -- presenting significant evidence of AGN activity. The confirmed dual AGN lie within $\sim3σ$ of the mm--X-ray correlation, suggesting this relation can be used to identify hidden pairs. By combining the position of each source relative to this correlation with independent star formation rate constraints, we propose a method to disentangle AGN and star formation contributions for sources with measured column densities. While our analysis is based on a small, heterogeneous local sample and relies on empirical scaling relations, these results indicate that millimeter continuum emission may provide a useful complementary diagnostic for obscured SMBH growth. ALMA observations at high angular resolutions are particularly valuable for this approach, while future facilities such as the ngVLA will be essential to test its robustness in larger and more distant samples.

Figures (2)

• Figure 1: Mm/x-ray luminosity versus SFR. Purple markers indicate the observed luminosity of the (U)LIRGs in our sample, plotted against their SFR derived from their IR luminosity. Light-blue lines show different SFR-mm/x-ray correlations. The grey dotted lines show a reference grid and are included solely to aid in the visual interpretation. (a) This panel compares the position of the (U)LIRGs sources in the SFR-X-ray (2-10 keV) against the correlations derived by Persic_Rephaeli_2007, derived from a sample of local star-forming galaxies, Mineo_2012, obtained from a sample of 29 nearby late-type galaxies, (U)LIRGs and star forming galaxies, and Lehmer_2010, measured from a sample of LIRGs at $z$$\sim$0. (b) Same as a) but for X-rays in the 14-150 keV range. (c) This panel shows the (U)LIRGs from our sample in the SFR-mm plane against the correlation derived by Yun_2002 derived from a sample of IR-selected dusty starburst galaxies and the correlation derived by Murphy_2012 , obtained from observations of nuclear and extra-nuclear star-forming regions in nearby galaxies.
• Figure 2: Observed peak millimeter continuum luminosity at 230 GHz versus X-ray luminosities in the 2–10 keV (top) and 14–150 keV bands (bottom). As in Figure \ref{['fig:1']}, the grey dotted lines show a reference grid and are included solely to aid in the visual interpretation. Grey circles show the isolated AGN sample from Kawamuro_2022, with their mm–X-ray relation plotted with a black line. Sources highlighted with black-edged markers correspond to single AGN in Kawamuro_2022 sample that also satisfy criterion (1) of our sample. Purple circles represent the GOALS-selected IR-bright galaxies observed at low resolution with ACA; confirmed dual AGNs are marked in red, and Arp 220 in orange. When available, X-ray luminosities are corrected for obscuration using the reported column densities (Tables \ref{['tab:results_1']} and \ref{['tab:results_2']}). For sources with $N_{\mathrm{H}}$ given as a range (Arp 220 E, IRASF 14348$-$1447, IRASF 17207$-$0014), a line of the same color indicates the shift in position if corrected for $N_{\mathrm{H}} \sim 10^{24}$ cm$^{-2}$ (first perpendicular bar) and $N_{\mathrm{H}} \sim 10^{25}$ cm$^{-2}$ (second perpendicular bar). The light-blue shaded region marks the area where emission can be entirely attributed to star formation, based on the SFR–mm relation from Yun_2002 and the SFR–X-ray relation from Persic_Rephaeli_2007, as described in Section \ref{['analisis: sfc']}. Light-blue lines indicate the expected mm and X-ray luminosities for systems with different AGN contributions at a given SFR.