X-ray properties and obscured fraction of AGN in the J1030 Chandra field^⋆

¹ Dipartimento di Fisica e Astronomia, Università di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
e-mail: matilde.signorini@unifi.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
³ University of California-Los Angeles, Department of Physics and Astronomy, 430 Portola Plaza, Los Angeles, CA 90095-1547, USA
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti, 93/3, 40129 Bologna, Italy
⁵ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634, USA
⁶ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁷ Department of Physics, University of Miami, Coral Gables, FL 33124, USA
⁸ Institut de Ciéncies del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franqués 1, 08028 Barcelona, Spain
⁹ ICREA, Pg. Luís Companys 23, 08010 Barcelona, Spain
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Scuola Internazionale Superiore di Studi Avanzati (SISSA), Via Bonomea 265, 34136 Trieste, Italy
¹² Istituto di Radioastronomia (IRA), Via Piero Gobetti 101, 40129 Bologna, Italy

Received: 9 March 2023
Accepted: 17 May 2023

Abstract

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.