$El\ Ascua$: An ultra-luminous, radio-loud, Compton-thick AGN at $z\sim 3.8$ hiding in plain sight revealed by MINERVA multi-band MIRI observations

Abstract

Heavily dust-obscured active galactic nuclei (AGN) are crucial for understanding supermassive black hole growth. Using multi-band MIRI photometry from the MINERVA survey, we report the discovery in the UDS field of an extremely mid-infrared bright Compton-thick AGN (CT AGN) at $z \sim 3.8$, \textit{el Ascua} (the Ember). The AGN and/or its host galaxy is detected at 870\,$μ$m with ALMA and at 1.4\,GHz with the VLA. With UV-to-radio spectral energy distribution modeling, we estimate that the dusty host galaxy has a stellar mass of $\log\left( M_\star / M_\odot \right)=10.82 \pm 0.57$ and a star formation rate of ${\rm{SFR}} =157 \pm 8 \,M_\odot\,{\rm yr^{-1}}$, and we confirm the presence of a powerful dusty AGN with an intrinsic accretion luminosity of $\log\left( L_{\rm{acc}} /{\rm erg\,s^{-1}} \right)=47.12 \pm 0.58$. CT obscuration is revealed by comparing the upper limit on the X-ray luminosity over $2-10 \,{\rm{keV}}$ with the $6 \,{\rmμ{m}}$ mid-IR AGN luminosity. We additionally find evidence for a radio excess by comparing its total IR-to-radio ratio to those of typical star-forming galaxies and radio-quiet AGN, implying that \textit{el Ascua} could be a rare radio-loud CT AGN. With an extremely high ratio between black hole accretion rate and SFR, the AGN is likely nearing the peak of a deeply obscured phase of black hole growth before AGN feedback blows away the obscuring material. Our work demonstrates the power of MIRI to reveal heavily obscured AGN missing from our supermassive black hole census.