By L. Baronchelli

Abstract: "To fully understand cosmic black hole growth, we need to constrain the population of heavily obscured active galactic nuclei (AGNs) at the peak of cosmic black hole growth (z ∼1–3). Sources with obscuring column densities higher than 10²⁴ atoms cm⁻², called Compton-thick (CT) AGNs, can be identified by excess X-ray emission at ∼20–30 keV, called the ‘Compton hump’. We apply the recently developed Spectral Curvature (SC) method to high-redshift AGNs (2 < z < 5) detected with Chandra. This method parametrizes the characteristic ‘Compton hump’ feature cosmologically redshifted into the X-ray band at observed energies <10 keV. We find good agreement in CT AGNs found using the SC method, and bright sources fit using their full spectrum with X-ray spectroscopy. In the Chandra Deep Field-South, we measure a CT fraction of 17 [+19,-11] % for sources with observed luminosity > 5 × 10⁴³ erg s⁻¹. In the Cosmological Evolution Survey (COSMOS), we find an observed CT fraction of 15 [+4,-3]% or 32 ± 11 % when corrected for the survey sensitivity. When comparing to low redshift AGNs with similar X-ray luminosities, our results imply that the CT AGN fraction is consistent with having no redshift evolution. Finally, we provide SC equations that can be used to find high-redshift CT AGNs (z > 1) for current (XMM–Newton) and future (eROSITA and ATHENA) X-ray missions."

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