Obscuration properties of mid-IR-selected AGN

Abstract

ABSTRACT The goal of this work is to study the obscuration properties of mid-infrared (mid-IR)-selected active galactic nuclei (AGN). For that purpose, we use Wide-field Infrared Survey Explorer (WISE) sources in the Stripe 82–XMM area to identify mid-IR AGN candidates, applying the Assef et al. criteria. Stripe 82 has optical photometry ≈2 times deeper than any single-epoch Sloan Digital Sky Survey (SDSS) region. XMM–Newton observations cover ∼26 deg2. Applying the aforementioned criteria, 1946 IR AGN are selected. ${\sim} 78{{\, \rm per\, cent}}$ have SDSS detection, while 1/3 of them are detected in X-rays, at a flux limit of $\rm {\sim} 5 \times 10^{-15}\, erg\, s^{-1}\, cm^{-2}$. Our final sample consists of 507 IR AGN with X-ray detection and optical spectra. Applying a $r-W2 > 6$ colour criterion, we find that the fraction of optically red AGN drop from 43 per cent for those sources with SDSS detection to $23{{\, \rm per\, cent}}$ for sources that also have X-ray detection. X-ray spectral fitting reveals 40 (${\sim} 8{{\, \rm per\, cent}}$) X-ray absorbed AGN ($N_\mathrm{ H} > 10^{22}\,{\rm cm}^{-2}$). Among the X-ray unabsorbed AGN, there are 70 red systems. To further investigate the absorption of these sources, we construct spectral energy distributions (SEDs) for the total IR AGN sample. SED fitting reveals that ${\sim} 20{{\, \rm per\, cent}}$ of the optically red sources have such colours because the galaxy emission is a primary component in the optical part of the SED, even though the AGN emission is not absorbed at these wavelengths. SED fitting also confirms that $12{{\, \rm per\, cent}}$ of the X-ray unabsorbed IR AGN are optically obscured.