X-ray luminosity function of Compton-thick active galactic nuclei in the early Universe (z ≥ 3)

Robustness and biases of the CTK population

IAASARS, National Observatory of Athens, Ioannou Metaxa and Vasileos Pavlou, GR-15236 Athens, Greece

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Received: 7 November 2025
Accepted: 8 June 2026

Abstract

The population of Compton-thick (CTK) active galactic nuclei (AGNs) represents a critical yet elusive phase in the growth of supermassive black holes (SMBHs). Constraining their abundance and evolution at high redshift is essential for understanding both SMBH growth and the origin of the cosmic X-ray background. We investigate the X-ray luminosity function (XLF) of CTK AGNs at z ≥ 3 using one of the largest available samples of X-ray-selected AGNs at high redshifts, containing 811 sources from XMM-Newton XXL-N and Chandra CCLS and CDF-S/N surveys. We first selected a subsample of ten high-probability CTK candidates, identified through Bayesian X-ray spectral fitting. Their multiwavelength properties are examined through spectral energy distribution modelling to assess the robustness of their CTK classification. For most sources, the inferred X-ray luminosities – and consequently their hydrogen column densities (N_H) – appear overestimated when compared with their infrared (IR) luminosities. After updating the N_H posteriors with IR-informed priors, only three sources remained consistent with the CTK regime. To compute the XLF for the entire CTK AGN population, we used 24 μm photometry to estimate IR luminosities and update the X-ray posteriors for all the remaining sources. Incorporating IR priors systematically reduces the inferred CTK number densities, yielding a more conservative and physically consistent estimate of the X-ray luminosity and absorption functions. We find that CTK AGNs constitute 17⁺¹²₋₁₁% of the total AGN population at 3 ≤ z ≤ 6, consistent with results at lower redshifts. Our analysis reveals no statistically significant evolution in the CTK fraction up to z ∼ 6, suggesting that the most heavily obscured accretion phase remains a persistent component of black hole growth throughout cosmic history. While the overall obscured AGN fraction (N_H > 10²³ cm⁻²) increases toward higher redshifts, the stable CTK fraction supports the interpretation that, at these epochs, the interstellar medium in typical host galaxies cannot produce CTK levels of obscuration.