Direct VLBI evidence for a buried AGN in the triple-merger LIRG UGC 2369S

Abstract

UGC 2369S is a luminous infrared galaxy (LIRG) undergoing a late-stage merger in a triple system, where the heavily obscured northern core is suspected to host an active galactic nucleus (AGN). However, severe dust and gas obscuration makes definitive confirmation challenging. We aim to provide direct observational evidence for the buried AGN through high-resolution radio imaging, while investigating the AGN accretion and feedback properties within this merger-driven gas-rich environment. We analyzed archival European VLBI Network (1.6 GHz) and Very Long Baseline Array (1.7 and 5 GHz) data of UGC 2369S. Through high-resolution imaging and visibility-domain Gaussian modeling, we characterized the morphology and intensity of its milliarcsecond-scale radio emission. A compact radio component is detected at the northern core, exhibiting high brightness temperature ($T_{\rm b}>10^7$ K) and flat radio spectrum ($α\approx -0.45$), which confirms the presence of an obscured AGN. The sub-Eddington accretion rate ($λ_{\rm Edd} \approx 2.7 \times 10^{-4}$) indicates that it falls within the radiatively inefficient accretion flow (RIAF) state. We provide direct imaging evidence for an AGN in the northern core of UGC 2369S, revealing a deeply buried, jet-emitting low-luminosity AGN (LLAGN) enshrouded by a Compton-thick gas cocoon. This demonstrates that VLBI is a uniquely effective tool for disentangling nuclear accretion and feedback processes within the heavily obscured environments of multiple-merger systems.