Minor Merger, Major Growth: An Overmassive, Highly Accreting Black Hole Powering a Secondary AGN In a Cosmic Noon Minor Merger

Abstract

We report the discovery of a spectroscopically confirmed z = 1.824 minor merger with a mass ratio of ~35:1 in which the secondary (smaller) galaxy hosts a luminous AGN. The system is identified in the 3D-HST survey and exhibits clear tidal features in James Webb Space Telescope imaging, confirming an ongoing interaction. Using archival Chandra X-ray observations, we detect 121 +/- 11 X-ray counts associated with the secondary galaxy, corresponding to a rest-frame 2-10 keV luminosity of L_X ~ (9 +/- 0.1) x 10^43 erg/s and a photon index of Gamma ~ 2.0-2.3. Analysis of the HST/WFC3 G141 grism spectrum yields an [O III] lambda5007 luminosity of (2 +/- 0.5) x 10^42 erg/s. Independent bolometric luminosity estimates from X-ray and [O III] emission are consistent, implying L_bol ~ (3-7) x 10^45 erg/s. Assuming standard black hole-galaxy scaling relations, the expected black hole mass is ~2 x 10^6 M_sun, which would require extreme super-Eddington accretion to explain the observed luminosity. On the other hand, assuming Eddington-limited or moderately sub-Eddington accretion implies a black hole mass more than an order of magnitude above expectations. The observed X-ray spectral slope disfavors low accretion rates, restricting the allowed parameter space to high lambda_Edd and elevated black hole masses. We conclude that the secondary AGN must be powered by an overmassive, highly accreting black hole, providing direct observational support for theoretical predictions that minor mergers can drive rapid black hole growth in secondary, smaller companions.