2019A&A...623A.172C

We present a multiwavelength study of seven active galactic nuclei (AGN) at spectroscopic redshift >2.5 in the 7Ms 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 7Ms Chandra spectra, we measured obscuring column densities N_H, X_ in excess of 7x10²²cm^–2 and intrinsic X-ray luminosities L_X>10⁴⁴erg/s for our targets, as well as equivalent widths for the Fe Kα emission line EW_rest≥0.5-1keV. 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_*∼10¹¹M_☉), IR luminosities (L_IR>10¹²L_☉), star formation rates (SFR∼190-1680M_☉/yr) and AGN bolometric luminosities (L_bol∼10⁴⁶erg/s) for our sample. We used an empirically calibrated relation between gas masses and FIR/submm luminosities and derived M_gas∼0.8-5.4x10¹⁰M_☉. 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_∼10^23–24cm^–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.