2023A&A...676A..49S

The 500ks Chandra ACIS-I observation of the field around the z = 6.31 quasar SDSS J1030+0524 is currently the fifth deepest extragalactic X-ray survey. The rich multi-band coverage of the field allowed an effective identification and redshift determination of the X-ray source counterparts; to date, a catalog of 243 extragalactic X-ray sources with either a spectroscopic or photometric redshift estimate in the range z ≈ 0 - 6 is available over an area of 355 arcmin². Given its depth and the multi-band information, this catalog is an excellent resource to investigate X-ray spectral properties of distant active galactic nuclei (AGN) and derive the redshift evolution of their obscuration. We performed a thorough X-ray spectral analysis for each object in the sample, and measured its nuclear column density N_H and intrinsic (de-absorbed) 2-10 keV rest-frame luminosity, L_2 - 10_. Whenever possible, we also used the presence of the Fe K_α emission line to improve the photometric redshift estimates. We measured the fractions of AGN hidden by column densities in excess of 10²² and 10²³ cm^–2 (f₂₂ and f₂₃, respectively) as a function of L_2 - 10_ and redshift, and corrected for selection effects to recover the intrinsic obscured fractions. At z ∼ 1.2, we found f₂₂ ∼ 0.7 - 0.8 and f₂₃ ∼ 0.5 - 0.6, respectively, in broad agreement with the results from other X-ray surveys. No significant variations in X-ray luminosity were found within the limited luminosity range probed by our sample (log L_2 - 10_ ∼ 42.8 - 44.3). When focusing on luminous AGN with log L_2 - 10_ ∼ 44 to maximize the sample completeness up to large cosmological distances, we did not observe any significant change in f₂₂ or f₂₃ over the redshift range z ∼ 0.8 - 3. Nonetheless, the obscured fractions we measure are significantly higher than is seen in the local Universe for objects of comparable intrinsic luminosity, pointing toward an increase in the average AGN obscuration toward early cosmic epochs, as also observed in other X-ray surveys. We finally compared our results with recent analytic models that ascribe the greater obscuration observed in AGN at high redshifts to the dense interstellar medium (ISM) of their hosts. When combined with literature measurements, our results favor a scenario in which the total column density of the ISM and the characteristic surface density of its individual clouds both increase toward early cosmic epochs as N_H, ISM_∝(1 + z)^δ^, with δ ∼ 3.3 - 4 and Σ_c, *_ ∝ (1 + z)², respectively.