Radio selection of heavily obscured AGN in the J1030 field: Unraveling a missing Compton-thick population

Context. Models of supermassive black holes (SMBHs) and galaxy coevolution, simulations, and recent JWST observations suggest that the population of heavily obscured, Compton-thick (CTK) active galactic nuclei (AGNs) at high redshift might be underestimated by X-ray surveys. To retrieve a complete census of SMBHs it is therefore necessary to identify new and complementary methods to select these sources, for example by exploiting the radio band as radio waves are almost unaffected by obscuration. Aims. We aim to test the effectiveness of radio selection to discover heavily obscured AGNs, particularly at high z, and, in turn, measure their abundance for the first time from a radio perspective. Methods. We considered the radio sources detected in the J1030 field, which is one of the fields with the deepest combination of 1.4 GHz radio and X-ray observations publicly available. We defined a radio excess parameter as the ratio between the star formation rate (SFR) that would correspond to the observed radio luminosity and the one directly derived from the spectral-energy-distribution (SED) fitting, REX = SFR 1.4GHz /SFR SED corr . We then selected sources with REX > 8.5 as radio-excess AGNs; this corresponds to a 3 σ excess above the median value (REX ~ 1). In this way, we find 145 radio-excess sources falling in the Chandra X-ray image footprint but without X-ray detection. Results. From the deep X-ray upper limits, we estimated a lower limit to the obscuration of each radio-excess AGN, finding on average log( N H /cm −2 ) > 23.7. A CTK AGN scenario is also supported by the results of the X-ray stacking analysis performed on sources at z > 1.5, which revealed X-ray luminosities and hardness ratios compatible with very highly obscured AGNs. Finally, we computed the number density of these radio-selected CTK AGNs. While at z ~ 2 the radio number density agrees well with the CTK AGN predictions of different population synthesis models, at z ~ 3 the radio selection returns a CTK AGN number density ~2–3 times larger than what is predicted by the Cosmic X-ray Background (CXB) models and X-ray observations. This result supports the effectiveness of radio emission in selecting the most obscured sources, unraveling a population of AGNs potentially missed by X-ray surveys at z > 3; this paves the way to a synergistic use of the future radio and X-ray facilities such as the Square Kilometer Array Observatory ( SKAO ), NewAthena , and the Advance X-ray Imager Satellite ( AXIS ).