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

Abstract

We present a multi-wavelength study of seven AGN at spectroscopic redshift > 2.5 in the 7 Ms Chandra Deep Field South, selected to have good FIR/sub-mm 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 × 10 22 cm −2 and intrinsic X-ray luminosities L X > 10 44 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 by using broadband photometry from CANDELS and Herschel catalogs. By means of an SED decomposition technique, we derived stellar masses (M * ∼ 10 11 M ), IR luminosities (L IR > 10 12 L ), star formation rates (SFR∼ 190 − 1680 M yr −1 ) and AGN bolometric luminosities (L bol ∼ 10 46 erg s −1 ) for our sample. We used an empirically-calibrated relation between gas masses and FIR/sub-mm luminosities and derived M gas ∼ 0.8 − 5.4 × 10 10 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, on the order of N H, ISM ∼ 10 23−24 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 kpc scales by the dense ISM in the host likely adds to or substitutes that produced by circumnuclear gas on pc scales (i.e., the torus of unified models). The lack of unobscured AGN among our ISM-rich targets supports this scenario.

Dust and gas content of high-redshift galaxies hosting obscured AGN in the Chandra Deep Field-South

“…This selection returns six targets spanning the redshift range ∼ 2.5 -4.8. This sample includes six of the seven sources studied by Circosta et al (2019), who adopted our same selection criteria except for the requirement of heavy obscuration. As pointed out by those authors, the fact that the FIR detection criterion is sufficient to select only obscured AGN indicates a connection between the SF-heated dust in the ISM that is responsible for the FIR to (sub)millimeter emission and the nuclear absorption.…”

“…This result was later confirmed by Mainieri et al (2005) who also reported a submillimeter detection at 850 µm using SCUBA. Exploiting high resolution (∼0.2") ALMA observations, Circosta et al (2019) found that the single object associated with XID 337 in the Spitzer/MIPS, Herschel and SCUBA maps is actually a blend of two sources, whose emission is dominated by a bright QSO at 3.5" separation; thus, the Mid-IR/Far-IR photometric datapoints of XID 337 were converted into upper limits. XID 551 was the first type II AGN ever detected in the CDF-S 1-Ms exposure at z>3 (Norman et al 2002), for which Comastri et al (2011) found a large column density (log N H > 23) performing an Xray spectrum analysis using 3.3-Ms XMM-Newton survey data.…”

“…Furthermore, we also added the measurements reported in this work at 2.1 mm. In addition to this, the other main difference with respect to Circosta et al (2019) is that they fit the FIR SED using several templates accounting for the stellar, star formation and AGN contributions, rather than a single MBB component. We discuss the consequences of these differences in Sec.…”

“…In this work we analyze for the first time the millimetric emission of the high-J CO emission and the dust continuum of six heavily obscured AGN (X-ray column densities N H,X > 10 23 cm −2 ) in the CDF-S at spectroscopic z > 2.5, included in the sample of Circosta et al (2019) and observed with ALMA. In Sec.…”

“…As for the gas mass, we explore several cases with different assumptions on the involved conversion factors, to address the large uncertainties in deriving such mass from both high-J CO transitions and dust long-wavelength emission. Then we derive the column densities for a simple spherical geometry and compare them with those measured from the X-ray spectra by Circosta et al (2019). We also calculate the dust mass in the optically-thin regime and the dynamical mass of the two sources for which the rotating disk model has been tested.…”