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

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}.

💡 Research Summary

This paper investigates the efficiency of radio selection for uncovering heavily obscured, Compton‑thick (CTK) active galactic nuclei (AGN) at high redshift, using the J1030 field, which benefits from exceptionally deep 1.4 GHz radio imaging and a 7 Ms Chandra X‑ray exposure. The authors define a radio‑excess parameter, REX = SFR₁.₄GHz / SFR_SED, where the star‑formation rate derived from the observed radio luminosity (SFR₁.₄GHz) is compared to the SFR obtained from multi‑wavelength spectral‑energy‑distribution (SED) fitting (SFR_SED). Sources with REX > 8.5, corresponding to a 3σ excess above the median REX ≈ 1 of the full radio sample, are classified as radio‑excess AGN candidates.

Applying this criterion to the radio catalogue yields 145 objects that lie within the Chandra footprint but lack individual X‑ray detections. For each source, the authors compute 3σ X‑ray flux upper limits and, assuming a typical AGN photon index (Γ = 1.8), infer a lower limit on the hydrogen column density. The average lower limit is log N_H (cm⁻²) > 23.7, indicating that the majority of the sample are heavily obscured, with many approaching the CTK regime (N_H ≥ 1.5 × 10²⁴ cm⁻²).

To strengthen the obscuration claim, a stacking analysis is performed on the 78 sources at z > 1.5. The stacked signal is detected at >5σ in the 0.5–7 keV band, with a hardness ratio (HR ≈ 0.3) and an average intrinsic 2–10 keV luminosity of ≈ 10⁴³․⁵ erg s⁻¹. These properties are consistent with a population of highly obscured AGN, where the observed X‑ray emission is dominated by reflected or scattered components.

The authors carefully assess possible contaminants: star‑formation‑driven radio emission, radio‑quiet AGN with modest excess, and spurious detections. By cross‑checking the far‑infrared–radio correlation, examining SED AGN fractions, and using simulations to estimate false‑positive rates (< 5 %), they argue that the majority of the sample are genuine AGN whose radio output is dominated by nuclear processes.

Number‑density calculations reveal that at z ≈ 2 (1.5 < z < 2.5) the radio‑selected CTK AGN space density (≈ 1.2 × 10⁻⁵ Mpc⁻³) agrees well with predictions from X‑ray background synthesis models (e.g., Gilli et al. 2007; Ueda et al. 2014). However, at z ≈ 3 (2.5 < z < 3.5) the radio‑derived density (≈ 3.5 × 10⁻⁵ Mpc⁻³) exceeds X‑ray‑based expectations by a factor of 2–3. This discrepancy suggests that current deep X‑ray surveys miss a substantial fraction of CTK AGN at early cosmic times, likely because column densities approaching log N_H ≈ 25 cm⁻² suppress even the hardest observed X‑ray photons.

The study demonstrates that radio excess selection is a powerful, complementary tool to X‑ray surveys for building a complete census of obscured SMBH growth, especially beyond z ≈ 3. The authors highlight the synergy with upcoming facilities: the Square Kilometre Array (SKA) will provide µJy‑level, wide‑area radio maps, while missions such as NewAthena and AXIS will deliver deeper, higher‑resolution X‑ray data. Joint analyses will enable precise measurements of the N_H distribution, Eddington ratios, and host‑galaxy properties of the hidden AGN population, thereby refining models of black‑hole–galaxy co‑evolution.

In summary, the paper introduces a robust radio‑excess metric, validates it through multi‑wavelength SED fitting and X‑ray stacking, and shows that radio selection uncovers a previously under‑represented CTK AGN population at high redshift, offering a vital pathway toward a more complete understanding of the cosmic growth of supermassive black holes.