The dependency of AGN infrared colour-selection on source luminosity and obscuration

Abstract

Aims. This work addresses the AGN IR-selection dependency on intrinsic source luminosity and obscuration, in order to identify and characterise biases that could affect conclusions in studies. Methods. We study IR-selected AGN in the Chandra Deep Field South (CDFS) survey and in the Cosmological Survey (COSMOS). The AGN sample is divided into low and high X-ray luminosity classes and into unobscured (type-1) and obscured (type-2) classes by means of X-ray and optical spectroscopy data. Specifically in the X-ray regime, we adopt the intrinsic luminosity taking the estimated column density (N H ) into account. We also take the opportunity to highlight important differences resulting from adopting different methods of assessing AGN obscuration. Results. In agreement with previous studies, we also find that AGN IR-selection efficiency shows a decrease with decreasing source AGN X-ray luminosity. For the intermediate-luminosity AGN population (43.3 < ∼ log(L X [erg s −1 ]) < ∼ 44), the efficiency also worsens with increasing obscuration (N H ). The same sample also shows an evolution with cosmic time of the obscured fraction at the highest X-ray luminosities, independently of the adopted type-1/type-2 classification method. Conclusions. We confirm that AGN IR-selection is genuinely biased towards unobscured AGNe, but only at intermediate luminosities.At the highest luminosities, where AGN IR-selection is more efficient, there is no obscuration bias. We show that type-1 AGNe are intrinsically more luminous than type-2 AGNe only at z < ∼ 1.6, thus resulting in more type-1 AGN being selected when the IR survey is shallower. Based on this and other studies, we conclude that deep hard-X-ray coverages, high-resolution IR imaging, or a combination of IR and radio data are required to recover the lower luminosity obscured AGN population. In addition, wide IR surveys are needed to recover the rare powerful, obscured AGN population. Finally, when the James Webb Space Telescope comes online, the broad-band filters 2.0 µm, 4.4 µm, 7.7 µm, and 18 µm will be essential for disentangling AGN from non-AGN dominated SEDs at depths where spectroscopy becomes impractical.

Active galactic nuclei as seen by the Spitzer Space Telescope

“…The completeness and reliability of the AGN selection varies according to the exact technique used. [89][90][91] In general, simple color-based techniques tend to fail at high redshifts (z>1.5), where the redshifted colors of the stellar populations of normal galaxies can cause them to fall into the selection regions ("wedges") of [78,79] (hereafter the "Lacy" and Stern" wedges, respectively), 81 In models when an unobscured AGN+torus model SED (blue dashed line in the left-hand panel) is embedded in a dusty host galaxy (red dot-dashed line), the mid-IR spectrum of the AGN is significantly altered (right panel). Dust processing changes the SED of the galaxy from the sum of the stars and AGN (black solid line in the left-hand panel).…”

“…For example, 97% of the AGNs found in 5 ks exposures with the Chandra X-ray Observatory in the X-Bootes field were identified in just 90 s of integration using IRAC, with 65% falling within the AGN selection criteria of Stern et al 106 In the optical, AGNs may be missing due to obscuration of the optical lines, or the presence of strong emission from star formation in the host disrupting the standard AGN emission line diagnostics. 90 When compared to X-ray studies, Spitzer was able to identify luminous ( ! (6 m) >6x10 44 erg/s/cm 2 ) Compton Thick quasars at z~2 that were not detectable in X-rays in 2-4 Ms observations with the Chandra X-ray Observatory, resulting in an estimated 24-48% fraction of such quasars that are Compton Thick, 107 consistent with recent estimates based on surveys with Chandra (e.g.…”

Luminous WISE-selected Obscured, Unobscured, and Red Quasars in Stripe 82∗

“…Therefore, in general, no single wavelength regime can be used to find all AGNs. Significant work has been done to explore biases and incompletenesses of AGN surveys selected at different wavelengths and flux limits (e.g., Eckart et al 2010;Juneau et al 2013;Mendez et al 2013;Messias et al 2014), arriving at a general consensus that infrared selection is most effective at identifying the most intrinsically luminous AGNs, which is the regime targeted in this work. We address the role of wide-field X-ray selection in Section 5.2.…”

Infrared Selection of Obscured Active Galactic Nuclei in the COSMOS Field

“…A proxy for obscuration can be inferred using the infrared to Xray luminosity ratio (or lower limit if there is no X-ray detection), where the AGN infrared luminosity can be estimated using state-of-the-art spectral energy distribution (SED) fitting techniques as discussed in Mullaney et al (2011); Kirkpatrick et al (2012Kirkpatrick et al ( , 2013; Dale et al (2014); Hernán-Caballero et al (2015); Kirkpatrick et al (2015). Obscured AGN can then be selected as sources that are under-luminous in the X-rays relative to their bolometric luminosity inferred from the infrared (Messias et al 2014).…”