Infrared Colours and Spectral Energy Distributions of Hard X-ray Selected Obscured and Compton-thick AGN

Abstract

We investigate infrared colours and spectral energy distributions (SEDs) of 338 X-ray selected AGN from Swift -BAT 105-month survey catalogue that have AKARI detection, in order to find a new selection criteria for Compton-thick AGN. By combining data from Galaxy Evolution Explore (GALEX ), Sloan Digital Sky Survey (SDSS) Data Release 14 (DR14), Two Micron All Sky Survey (2MASS), Wide-field Infrared Survey Explorer (WISE ), AKARI and Herschel for the first time we perform ultraviolet (UV) to far-infrared (FIR) SED fitting 158 Swift -BAT AGN by CIGALE and constrain the AGN model parameters of obscured and Compton-thick AGN. The comparison of average SEDs show while the mid-IR (MIR) SEDs are similar for the three AGN populations, optical/UV and FIR regions have differences. We measure the dust luminosity, the pure AGN luminosity and the total infrared (IR) luminosity. We examine the relationships between the measured infrared luminosities and the hard X-ray luminosity in the 14-195 keV band. We show that the average covering factor of Compton-thick AGN is higher compared to the obscured and unobscured AGN. We present a new infrared selection for Compton-thick AGN based on MIR and FIR colours ([9µm − 22µm] > 3.0 and [22µm − 90µm] < 2.7) from WISE and AKARI. We find two known Compton-thick AGN that are not included in the Swift -BAT sample, and conclude that MIR colours covering 9.7µm silicate absorption and MIR continuum can be a promising new tool to identify Compton-thick AGN.

BAT AGN Spectroscopic Survey-XXIII. A New Mid-Infrared Diagnostic for Absorption in Active Galactic Nuclei

“…However, it is evident from F 11 that these color cuts cannot reliably distinguish between unobscured, obscured, and CT AGNs, as the AGNs from these three different obscuration bins largely occupy the same [9 µm]-[22 µm] and [22 µm]-[90 µm] parameter space. In the case of Swift/BAT, the color-color criteria proposed by Kilerci-Eser et al (2020) yields a far lower success rate of identifying heavily obscured and CT AGNs than the criteria set forth in this work (e.g. Equations 7 and 8; see Tables 3, 5, and 6).…”

“…Of the candidates that have inferred or directly measured column densities in the literature (24/36), we find eight CT AGNs, six moderately obscured AGNs, two lightly obscured AGNs, and three unobscured AGN, while a remaining five AGNs have conflicting measurements of N H in the literature (all five of which have been reported as CT at least once in the past). 6 Therefore, we conclude that our diagnostic criteria proposed in Equations 5, 7, and 8 offer a more reliable method for identifying candidate CT AGNs than the mid-IR to far-IR color criteria proposed by Kilerci-Eser et al (2020).…”

“…5 The remainder of the infrared galaxy sample of Kilerci Eser & Goto ( 2018) is represented with blue contours in F 11 that partially overlap a significant number of Swift/BAT CT, obscured, and unobscured AGNs, suggesting that some portion of these infrared galaxies may in fact host heavily obscured AGNs. Despite finding a few cases of CT AGNs between the Swift/BAT and Kilerci Eser & Goto (2018) samples, the diagnostic criteria set forth in Kilerci-Eser et al (2020) does not appear to provide a complete or reliable (see Section 5.4 of Kilerci-Eser et al 2020) method of selecting CT AGNs.…”

“…As an additional comparison between our selection method and that proposed by Kilerci-Eser et al (2020), we turned our attention to the infrared galaxy catalog from Kilerci Eser & Goto (2018). We matched this sample to the AllWISE catalog and the 4XMM DR9 XMM-Newton Serendipitous Source Catalog (Webb et al submitted), using a match radius of 10 for each, which yielded a sample of 401 local (z < 0.1) infrared galaxies with XMM-Newton and WISE detections.…”

“…(2018) (with the understanding that some heavily obscured AGNs will be missed with this simple approach), and in all four cases we recover a significant number of candidate heavily obscured or CT AGNs. We show mid-IR AGNs selected as a result of the Stern et al (2012) cut in Figure 11 (blue squares); these candidate CT AGNs likely inhabited the blue contoured prominence that overlapped the Swift/BAT AGNs in F 11, but were missed due to fact that they did not satisfy the criteria proposed by Kilerci-Eser et al (2020). Table 7 in the Appendix lists the 36 candidate CT AGNs selected from Kilerci Eser & Goto (2018) using the L 22 µm /L 4.6 µm diagnostic region and at least one of the mid-IR selection cuts listed above.…”