Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey

Abstract

Aims. Our goal is to probe the populations of obscured and unobscured AGN investigating their optical-IR and X-ray properties as a function of X-ray flux, luminosity and redshift within a hard X-ray selected sample with wide multiwavelength coverage. Methods. We selected a sample of 136 X-ray sources detected at a significance of ≥3σ in the 2−10 keV band (F 2−10 > ∼ 10 −14 erg cm −2 s −1 ) in a ∼1 deg 2 area in the XMM Medium Deep Survey (XMDS). The XMDS area is covered with optical photometry from the VVDS and CFHTLS surveys and infrared Spitzer data from the SWIRE survey. Based on the X-ray luminosity and X-ray to optical ratio, 132 sources are likely AGN, of which 122 have unambiguous optical -IR identification. The observed optical and IR spectral energy distributions of all identified sources are fitted with AGN/galaxy templates in order to classify them and compute photometric redshifts. X-ray spectral analysis is performed individually for sources with a sufficient number of counts and using a stacking technique for subsamples of sources at different flux levels. Hardness ratios are used to estimate X-ray absorption in individual weak sources. Results. 70% of the AGN are fitted by a type 2 AGN or a star forming galaxy template. We group them together in a single class of "optically obscured" AGN. These have "red" optical colors and in about 60% of cases show significant X-ray absorption (N H > 10 22 cm −2 ). Sources with SEDs typical of type 1 AGN have "blue" optical colors and exhibit X-ray absorption in about 30% of cases. The stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and has an average spectral slope of Γ = 1.6. The subsample of objects fitted by a star forming galaxy template has an even harder stacked spectrum, with Γ ∼ 1.2−1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower luminosities. X-ray absorption is less common than "optical" obscuration and its incidence is nearly constant with redshift and luminosity. This implies that at high luminosities X-ray absorption is not necessarily related to optical obscuration. The estimated surface densities of obscured, unobscured AGN and type 2 QSOs are respectively 138, 59 and 35 deg −2 at F > 10 −14 erg cm −2 s −1 .

BAT AGN Spectroscopic Survey. VIII. Type 1 AGN with Massive Absorbing Columns

“…Lowering the threshold for X-ray absorbed AGNs to log N H =21.5 cm −2 increases the frequency to 20%, so we can confidently say the X-ray absorbed fraction within Type 1 AGNs is between 10% and 20%. These fractions are more in agreement with the results of Perola et al (2004) and nearly a factor of 3 smaller than the rates found by Tajer et al (2007) and Merloni et al (2014). What is further clear from this study is that the hydrogen column densities measured from the X-ray spectra are generally much larger than those measured from the BLR extinction, assuming a Galactic GDR.…”

“…Under the simple picture of a static dusty torus around an AGN, both optical and X-ray measurements of the gas column density should agree. To a large extent this is true, as many studies find that Type 1 AGNs show little to no X-ray absorption, while most Type 2 AGNs are X-ray absorbed with N H 10 22 cm −2 (e.g., Smith & Done 1996;Turner et al 1997;Risaliti et al 1999;Garcet et al 2007;Mainieri et al 2007;Tajer et al 2007;Antonucci 2012;Malizia et al 2012;Merloni et al 2014;Davies et al 2015) in accordance with the unified model. Of course, the unified model, while broadly successful in explaining the diversity of AGNs, is simplified and investigations of differences between Type 1 and Type 2 AGNs that cannot be explained by this paradigm can help to reveal the complex nature of AGNs.…”

“…Perola et al (2004) found that 10% of broad line AGNs are X-ray absorbed within the HELLAS2XMM 1 degree field survey, while Tozzi et al (2006) estimated that at least 20% of AGNs in the Chandra Deep Field South have inconsistent optical and X-ray classifications. Both Tajer et al (2007) and, more recently, Merloni et al (2014) instead find that around 30% of optically unobscured AGNs are X-ray absorbed. Merloni et al (2014), interestingly, also showed an increasing fraction of X-ray absorbed, but optically unobscured AGNs at higher X-ray luminosities.…”

The dependence of X-ray AGN activity on host galaxy properties and environment

“…Figure 4 shows the distribution of HR for the sources selected as Type-1 and Type-2 as described above. As expected, these results indicate that the selected Type-2 AGN have a higher hardness ratios than the Type-1 AGN population, due to their higher column densities (Tajer et al 2007). Sazonov & Revnivtsev (2004) have estimated column densities from the flux ratio F [8−20] /F [3−8] , where F [8−20] and F [3−8] are the flux measurements in the [8][9][10][11][12][13][14][15][16][17][18][19][20] and [3][4][5][6][7][8] keV X-ray bands respectively.…”

“…2.1). Following Tajer et al (2007), we define the hardness ratio as HR = (CR H − CR S )/(CR H + CR S ), where CR H and CR S are the count rates in the hard [2-10] and soft [0.5-2] keV band, respectively. Figure 4 shows the distribution of HR for the sources selected as Type-1 and Type-2 as described above.…”