X-ray emission of z > 2.5 active galactic nuclei can be obscured by their host galaxies

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
e-mail: ccircost@eso.org
² Dipartimento di Fisica e Astronomia dell’Università degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
³ INAF/OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129 Bologna, Italy
⁴ Univ. Lyon, Univ. Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁵ Instituto de Astrofisica and Centro de Astroingenieria, Facultad de Fisica, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 22, Chile
⁶ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100012, PR China
⁷ INAF, Istituto di Radioastronomia – Italian ARC, Via Piero Gobetti 101, 40129 Bologna, Italy
⁸ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

Received: 14 October 2018
Accepted: 16 January 2019

Abstract

We present a multiwavelength study of seven active galactic nuclei (AGN) at spectroscopic redshift >2.5 in the 7 Ms Chandra Deep Field South that were selected for their good far-infrared (FIR) and submillimeter (submm) detections. Our aim is to investigate the possibility that the obscuration observed in the X-rays can be produced by the interstellar medium (ISM) of the host galaxy. Based on the 7 Ms Chandra spectra, we measured obscuring column densities N_H,  X in excess of 7 × 10²² cm⁻² and intrinsic X-ray luminosities L_X >  10⁴⁴ erg s⁻¹ for our targets, as well as equivalent widths for the Fe Kα emission line EW_rest ≳ 0.5−1 keV. We built the UV-to-FIR spectral energy distributions (SEDs) by using broadband photometry from the CANDELS and Herschel catalogs. By means of an SED decomposition technique, we derived stellar masses (M_* ∼ 10¹¹ M_⊙), IR luminosities (L_IR >  10¹² L_⊙), star formation rates (SFR ∼ 190−1680 M_⊙ yr⁻¹) and AGN bolometric luminosities (L_bol ∼ 10⁴⁶ erg s⁻¹) for our sample. We used an empirically calibrated relation between gas masses and FIR/submm luminosities and derived M_gas ∼ 0.8−5.4 × 10¹⁰ M_⊙. High-resolution (0.3−0.7″) ALMA data (when available, CANDELS data otherwise) were used to estimate the galaxy size and hence the volume enclosing most of the ISM under simple geometrical assumptions. These measurements were then combined to derive the column density associated with the ISM of the host, which is on the order of N_H,  ISM ∼ 10²³⁻²⁴ cm⁻². The comparison between the ISM column densities and those measured from the X-ray spectral analysis shows that they are similar. This suggests that at least at high redshift, significant absorption on kiloparsec scales by the dense ISM in the host likely adds to or substitutes that produced by circumnuclear gas on parsec scales (i.e., the torus of unified models). The lack of unobscured AGN among our ISM-rich targets supports this scenario.