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

M. Droguett-Callejas, E. Treister, L. Barcos-Muñoz, M. Johnstone, F. E. Bauer, T. Kawamuro, N. Torres-Albà, C. Ricci, M. Koss, Y. Song, A. Peca, A. Evans, J. González

First listed 2026-01-21 | Last updated 2026-02-25

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.

Short digest

Tests whether the millimeter (~200 GHz, 1.3 mm)–hard X-ray (14–150 keV) luminosity correlation can reveal buried SMBH activity in local (U)LIRG mergers using archival ALMA continuum for GOALS systems plus the confirmed dual-AGN UGC 4211. The sample yields three sources—one confirmed AGN and two strong candidates—with the confirmed dual AGN lying within ~3σ of the mm–X-ray relation, consistent with accretion-powered mm emission even at Compton-thick columns. Offsets from the correlation combined with independent star-formation rate constraints (and NH where available) provide a practical way to apportion AGN vs. star-formation contributions. While based on a small, heterogeneous local set and empirical scalings, the work argues that compact mm continuum is a dust-insensitive diagnostic for hidden SMBH growth; ALMA resolves nearby nuclei now and ngVLA will scale this to larger, more distant samples.

Key figures to inspect

• mm (∼200 GHz) luminosity vs. hard X-ray (14–150 keV) luminosity with the best-fit relation and 1–3σ bands; highlight positions of confirmed dual AGN and the two candidate AGN to see who lands within ~3σ and who deviates.
• ALMA 1.3 mm continuum maps of late-stage mergers (including UGC 4211), showing compact nuclear peaks and dual-nucleus separations; compare with X-ray centroid marks to assess astrometric alignment and plausibility of AGN-powered mm cores.
• Diagnostic panel using each source’s offset from the mm–X-ray correlation combined with independent SFR limits to infer the AGN vs. star-formation share of the mm flux; look for cases where SF alone cannot explain the mm luminosity.
• Column density NH versus offset from the mm–X-ray relation to evaluate how Compton-thick candidates behave relative to the correlation and whether extreme NH systematically shifts sources.
• Summary table/plot of sample definition and ALMA setup (programs 2016.2.00055.S and 2017.1.00767.S): beam sizes and continuum fluxes per nucleus, to judge whether the mm emission is compact enough to be nuclear rather than extended SF.