Inferring Compton-thick AGN candidates at z > 2 with Chandra using the >8 keV rest-frame spectral curvature

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|$^{+19}_{-11}\hbox{\,per\,cent}$| (3/17) for sources with observed luminosity >5 × 1043erg s−1. In the Cosmological Evolution Survey (COSMOS), we find an observed CT fraction of |$15^{+4}_{-3}\hbox{\,per\,cent}$| (40/272) or 32 ± 11 per cent 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.