Radio selection of heavily obscured AGN in the J1030 field: unraveling a missing Compton-thick population
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2026
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| author | Mazzolari, Giovanni Gilli, Roberto Mignoli, Marco Brusa, Marcella Prandoni, Isabella Vito, Fabio Delvecchio, Ivan Lanzuisi, Giorgio Peca, Alessandro Comastri, Andrea Marchesi, Stefano Chiaberge, Marco Brienza, Marisa Vignali, Cristian Signorini, Matilde D'Amato, Quirino Gentile, Fabrizio Iwasawa, Kazushi Norman, Colin Traina, Alberto Loiacono, Federica Baldini, Pietro Annunziatella, Marianna Decarli, Roberto |
| author_facet | Mazzolari, Giovanni Gilli, Roberto Mignoli, Marco Brusa, Marcella Prandoni, Isabella Vito, Fabio Delvecchio, Ivan Lanzuisi, Giorgio Peca, Alessandro Comastri, Andrea Marchesi, Stefano Chiaberge, Marco Brienza, Marisa Vignali, Cristian Signorini, Matilde D'Amato, Quirino Gentile, Fabrizio Iwasawa, Kazushi Norman, Colin Traina, Alberto Loiacono, Federica Baldini, Pietro Annunziatella, Marianna Decarli, Roberto |
| contents | We tested the effectiveness of radio selection to discover heavily obscured AGNs, particularly at high-z, and we measured their abundance for the first time from a radio perspective. We consider 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. 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, $\rm REX=SFR_{1.4GHz}/SFR^{corr}_{SED}$. We then select as radio excess AGN those sources with $\rm REX>8.5$, corresponding to a $3σ$ excess above the median value. In this way, we find 145 radio-excess sources falling into the \textit{Chandra} X-ray image footprint but without X-ray detection. From the deep X-ray upper limits, we estimated a lower limit to the obscuration of each radio-excess AGN, finding on average $\log (N_H/\rm{cm^{-2}})>23.7$. 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 AGN. Finally, we computed the number density of these radio-selected CTK AGN. While at $z\sim 2$ the radio number density agrees well with the CTK AGN predictions of different population synthesis models, at $z\sim3$ the radio selection returns a CTK AGN number density $\sim 2-3$ times larger than what is predicted by the X-ray models and observations. This result supports the effectiveness of radio emission in selecting the most obscured sources, unraveling a population of AGN potentially missed by X-rays surveys at $z>3$, paving the way to a synergistic use of the future radio and X-ray facilities such as the \textit{SKAO} and \textit{NewAthena}. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2603_10176 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Radio selection of heavily obscured AGN in the J1030 field: unraveling a missing Compton-thick population Mazzolari, Giovanni Gilli, Roberto Mignoli, Marco Brusa, Marcella Prandoni, Isabella Vito, Fabio Delvecchio, Ivan Lanzuisi, Giorgio Peca, Alessandro Comastri, Andrea Marchesi, Stefano Chiaberge, Marco Brienza, Marisa Vignali, Cristian Signorini, Matilde D'Amato, Quirino Gentile, Fabrizio Iwasawa, Kazushi Norman, Colin Traina, Alberto Loiacono, Federica Baldini, Pietro Annunziatella, Marianna Decarli, Roberto Astrophysics of Galaxies Cosmology and Nongalactic Astrophysics We tested the effectiveness of radio selection to discover heavily obscured AGNs, particularly at high-z, and we measured their abundance for the first time from a radio perspective. We consider 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. 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, $\rm REX=SFR_{1.4GHz}/SFR^{corr}_{SED}$. We then select as radio excess AGN those sources with $\rm REX>8.5$, corresponding to a $3σ$ excess above the median value. In this way, we find 145 radio-excess sources falling into the \textit{Chandra} X-ray image footprint but without X-ray detection. From the deep X-ray upper limits, we estimated a lower limit to the obscuration of each radio-excess AGN, finding on average $\log (N_H/\rm{cm^{-2}})>23.7$. 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 AGN. Finally, we computed the number density of these radio-selected CTK AGN. While at $z\sim 2$ the radio number density agrees well with the CTK AGN predictions of different population synthesis models, at $z\sim3$ the radio selection returns a CTK AGN number density $\sim 2-3$ times larger than what is predicted by the X-ray models and observations. This result supports the effectiveness of radio emission in selecting the most obscured sources, unraveling a population of AGN potentially missed by X-rays surveys at $z>3$, paving the way to a synergistic use of the future radio and X-ray facilities such as the \textit{SKAO} and \textit{NewAthena}. |
| title | Radio selection of heavily obscured AGN in the J1030 field: unraveling a missing Compton-thick population |
| topic | Astrophysics of Galaxies Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2603.10176 |