The XXL Survey

XL. Obscuration properties of red AGNs in XXL-N

¹ National Observatory of Athens, V. Paulou & I. Metaxa, 15 236 Penteli, Greece
e-mail: vmasoura@noa.gr
² Section of Astrophysics, Astronomy and Mechanics, Department of Physics, Aristotle University of Thessaloniki, 54 124 Thessaloniki, Greece
³ Instituto de Fisica de Cantabria (CSIC-Universidad de Cantabria), Avenida de los Castros, 39005 Santander, Spain
⁴ Dipartimento di Fisica e Astronomia, Alma Mater Studiorum, Universitá degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁶ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
⁷ INAF, IASF Milano, Via Corti 12, 20133 Milano, Italy
⁸ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK
⁹ Department of Astronomy, University of Geneva, Ch. d’Écogia 16, 1290 Versoix, Switzerland

Received: 21 November 2019
Accepted: 9 March 2020

Abstract

The combination of optical and mid-infrared (MIR) photometry has been extensively used to select red active galactic nuclei (AGNs). Our aim is to explore the obscuration properties of these red AGNs with both X-ray spectroscopy and spectral energy distributions (SEDs). In this study, we re-visit the relation between optical/MIR extinction and X-ray absorption. We use IR selection criteria, specifically the W1 and W2 WISE bands, to identify 4798 AGNs in the XMM-XXL area (∼25 deg²). Application of optical/MIR colours (r−W2 >  6) reveals 561 red AGNs (14%). Of these, 47 have available X-ray spectra with at least 50 net (background-subtracted) counts per detector. For these sources, we construct SEDs from the optical to the MIR using the CIGALE code. The SED fitting shows that 44 of these latter 47 sources present clear signs of obscuration based on the AGN emission and the estimated inclination angle. Fitting the SED also reveals ten systems (∼20%) which are dominated by the galaxy. In these cases, the red colours are attributed to the host galaxy rather than AGN absorption. Excluding these ten systems from our sample and applying X-ray spectral fitting analysis shows that up to 76% (28/37) of the IR red AGNs present signs of X-ray absorption. Thus, there are nine sources (∼20% of the sample) that although optically red, are not substantially X-ray absorbed. Approximately 50% of these sources present broad emission lines in their optical spectra. We suggest that the reason for this apparent discrepancy is that the r−W2 criterion is sensitive to smaller amounts of obscuration relative to the X-ray spectroscopy. In conclusion, it appears that the majority of red AGNs present considerable obscuration levels as shown by their SEDs. Their X-ray absorption is moderate with a mean of N_H ∼ 10²² cm⁻².