Large-scale clustering of buried X-ray AGN: Trends in AGN obscuration and redshift evolution

¹ Department of Physics, University of Helsinki, PO Box 64 00014 Helsinki, Finland
e-mail: akke.viitanen@helsinki.fi
² INAF-Osservatorio astronomico di Capodimonte, Via Moiariello 16, 30131 Naples, Italy
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁴ School of Physics & Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ UK
⁵ INAF-Osservatorio di Astrofisica e Scienza delle Spazio di Bologna, OAS, Via Gobetti 93/3, 40129 Bologna, Italy

Received: 28 October 2022
Accepted: 4 April 2023

Abstract

Aims. In order to test active galactic nucleus (AGN) unification and evolutionary models, we measured the AGN clustering properties as a function of AGN obscuration defined in terms of hydrogen column density, N_H. In addition to measuring the clustering of unobscured (N_H < 10²² cm⁻²) and moderately obscured (10²² ≤ N_H < 10^23.5) AGNs, we also targeted highly obscured sources (N_H ≥ 10^23.5) up to redshifts of z = 3.

Methods. We have compiled one of the largest samples of X-ray-selected AGNs from a total of eight deep XMM/Chandra and multiwavelength surveys. We measured the clustering as a function of both AGN obscuration and redshift using the projected two-point correlation function, w_p(r_p). We modeled the large-scale clustering signal, measured the AGN bias, b(z, N_H), and interpreted it in terms of the typical AGN host dark matter halo, M_halo(z, N_H).

Results. We find no significant dependence of AGN clustering on obscuration, suggesting similar typical masses of the hosting halos as a function of N_H. This result matches expectations of AGN unification models, in which AGN obscuration depends mainly on the viewing angle of the obscuring torus. We measured, for the first time, the clustering of highly obscured AGNs and find that these objects reside in halos with typical mass log M_halo = 12.98_−0.22^+0.17[h⁻¹ M^⊙] (12.28_−0.19^+0.13) at low z ∼ 0.7 (high z ∼ 1.8) redshifts. We find that irrespective of obscuration, an increase in AGN bias with redshift is slower than the expectation for a constant halo mass and instead follows the growth rate of halos, known as the passive evolution track. This implies that for those AGNs the clustering is mainly driven by the mass growth rate of the hosting halos and galaxies across cosmic time.