The incidence of obscuration in active galactic nuclei

Abstract

We study the incidence of nuclear obscuration on a complete sample of 1310 AGN selected on the basis of their rest-frame 2-10 keV X-ray flux from the XMM-COSMOS survey, in the redshift range 0.3 < z < 3.5. We classify the AGN as obscured or un-obscured on the basis of either the optical spectral properties and the overall SED or the shape of the X-ray spectrum. The two classifications agree in about 70% of the objects, and the remaining 30% can be further subdivided into two distinct classes: at low luminosities X-ray un-obscured AGN do not always show signs of broad lines or blue/UV continuum emission in their optical spectra, most likely due to galaxy dilution effects; at high luminosities broad line AGN may have absorbed X-ray spectra, which hints at an increased incidence of small-scale (subparsec) dust-free obscuration. We confirm that the fraction of obscured AGN is a decreasing function of the intrinsic X-ray luminosity, while the incidence of absorption shows significant evolution only for the most luminous AGN, which appear to be more commonly obscured at higher redshift. We find no significant difference between the mean stellar masses and star formation rates of obscured and un-obscured AGN hosts. We conclude that the physical state of the medium responsible for obscuration in AGN is complex, and mainly determined by the radiation environment (nuclear luminosity) in a small region enclosed within the gravitational sphere of influence of the central black hole, but is largely insensitive to the wider scale galactic conditions.

A new sample of X-ray selected narrow emission-line galaxies

“…Both the normal Sy2 (91%) and unobscured Sy2 (83%) are mainly galaxy dominated as expected by Merloni et al (2014) and shown in Fig. 8.…”

“…An N H of 4 × 10 21 cm −2 was used as the separation between X-ray obscured and unobscured AGN (Caccianiga et al 2007;Merloni et al 2014). In Fig.…”

“…Thus the dilution by the galaxy may explain why, even if the AGN is observed directly and so is not obscured by the torus, no BEL are observed (Merloni et al 2014).…”

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

“…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.…”