Identifying Luminous Active Galactic Nuclei in Deep Surveys: Revised Irac Selection Criteria

Abstract

Spitzer IRAC selection is a powerful tool for identifying luminous AGN. For deep IRAC data, however, the AGN selection wedges currently in use are heavily contaminated by star-forming galaxies, especially at high redshift. Using the large samples of luminous AGN and high-redshift star-forming galaxies in COSMOS, we redefine the AGN selection criteria for use in deep IRAC surveys. The new IRAC criteria are designed to be both highly complete and reliable, and incorporate the best aspects of the current AGN selection wedges and of infrared power-law selection while excluding high redshift star-forming galaxies selected via the BzK, DRG, LBG, and SMG criteria. At QSO-luminosities of log L 2−10keV (ergs s −1 )≥ 44, the new IRAC criteria recover 75% of the hard X-ray and IRAC-detected XMM-COSMOS sample, yet only 38% of the IRAC AGN candidates have X-ray counterparts, a fraction that rises to 52% in regions with Chandra exposures of 50-160 ks. X-ray stacking of the individually X-ray non-detected AGN candidates leads to a hard X-ray signal indicative of heavily obscured to mildly Compton-thick obscuration (log N H (cm −2 ) = 23.5 ± 0.4). While IRAC selection recovers a substantial fraction of luminous unobscured and obscured AGN, it is incomplete to lowluminosity and host-dominated AGN.

On R − W1 as a Diagnostic to Discover Obscured Active Galactic Nuclei in Wide-Area X-Ray Surveys

“…Identifying AGNs based on infrared-only colors has the highest reliability at brighter fluxes (e.g., Barmby et al 2006;Cardamone et al 2008;Donley et al 2008Donley et al , 2012Eckart et al 2010;Mendez et al 2013). As we show in Figure 17 To answer this, we again plot the distribution of R W1 -for the different luminosity bins in Figure 18 and is an efficient and robust diagnostic to select unobscured and obscured AGNs, including Comptonthick AGNs that can be missed altogether by X-ray surveys, additional metrics, such as R W1 -, are useful in recovering a reddened population missed by this infrared-only cut (Stern et al 2012;Yan et al 2013;Donoso et al 2014).…”

Cold dust emission from X-ray AGN in the SCUBA-2 Cosmology Legacy Survey: dependence on luminosity, obscuration and AGN activity

“…As illustrated in Fig. 7, UltraVISTA galaxies that satisfy the Donley et al (2012) AGN selection criteria, occupy the entire redshift-mass plane and do not dominate at high masses. In this high-mass regime, the UltraVISTA galaxies are therefore unlikely to suffer from heavy contamination from highly obscured AGN.…”

“…However, before proceeding it is important to highlight the selection biases and incompletenesses of an AGN catalogue selected in this way. Donley et al (2012) have Table 1. Summary of X-ray AGN with potential 850-µm bright identifications in the COSMOS field.…”

Dusty star-forming galaxies at high redshift

“…These were recently revisited by Donley et al (2012), under the premise that the previously developed color-selection "wedges" were contaminated by star-forming galaxies given deep enough IRAC data.…”