The $XXL$ survey: XL. Obscuration properties of red AGN in $XXL-N$

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 ($\sim 25$deg${}^2$). 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 ($\sim 20\%$) 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 ($\sim 20\%$ 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 ${\mathrm{N}}_{\mathrm{H}}\sim 10^{22}\mathrm{c}{\mathrm{m}}^{-2}$.

A&A accepted, 37 pages, 51 figures, 7 tables

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• XXL-HSC: Host properties of X-ray detected AGNs in XXL clusters
• The incidence of X-ray AGN and non-AGN galaxies in the far-infrared: insights into host galaxy properties and AGN obscuration
• AGN obscuration in optical and X-rays: Host properties and the interplay of nuclear and galactic gas and dust in a combined SDSS-XMM sample
• Nuclear Activity and Host Galaxy Properties of Low-Luminosity AGN Identified from VLA Observations