Mid‐Infrared Selection of Active Galaxies

Abstract

Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ultraviolet to mid-infrared (k P 5 m) continuum of stellar populations is dominated by the composite blackbody curve and peaks at approximately 1.6 m, the ultraviolet to mid-infrared continuum of active galactic nuclei (AGNs) is dominated by a power law. Consequently, with a sufficient wavelength baseline, one can easily distinguish AGNs from stellar populations. Mirroring the tendency of AGNs to be bluer than galaxies in the ultraviolet, where galaxies (and stars) sample the blue, rising portion of stellar spectra, AGNs tend to be redder than galaxies in the mid-infrared, where galaxies sample the red, falling portion of the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors, derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically identified sources from the AGN and Galaxy Evolution Survey. On the basis of this spectroscopic sample, we find that simple mid-infrared color criteria provide remarkably robust separation of active galaxies from normal galaxies and Galactic stars, with over 80% completeness and less than 20% contamination. Considering only broad-lined AGNs, these mid-infrared color criteria identify over 90% of spectroscopically identified quasars and Seyfert 1 galaxies. Applying these color criteria to the full imaging data set, we discuss the implied surface density of AGNs and find evidence for a large population of optically obscured active galaxies.

Dusty star-forming galaxies at high redshift

“…Motivated by the fact that X-rays can be absorbed in high column density environments, a number of groups developed mid-IR color selection techniques to identify AGN at high-z utilizing the bandpasses available on the Spitzer Space Telescope Lacy et al (2004) and Stern et al (2005). 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.…”

STAR-FORMING BRIGHTEST CLUSTER GALAXIES AT 0.25 < z < 1.25: A TRANSITIONING FUEL SUPPLY

“…Color corrections from Stern et al (2012) have been applied to match data from these telescopes to a common photometric system. Following Stern et al (2005Stern et al ( , 2012, we classify systems with mid-IR colors 3.5 4.5 0 [ ]-[ ] > as AGN, while those with redder colors are either passive or star-forming galaxies. In this color space, the Phoenix cluster (McDonald et al 2012a) is the only BCG harboring a strong AGN.…”

Ultra Steep Spectrum Radio Sources in the Lockman Hole: SERVS Identifications and Redshift Distribution at the Faintest Radio Fluxes

“…This allows for the use of IR colour diagnostics plots, which may offer further clues about the nature of these sources. Figure 4 shows the location of radio sources in the Lockman Hole with appropriate IR detections in two of the most commonly used IR diagnostic diagrams (Lacy et al 2004;Stern et al 2005) together with the recent KI diagnostic diagram of Messias et al (2011). USS sources show a wide range of IR colours, with a significant fraction being associated with AGN coloursamong those USS sources with simultaneous detections at the relevant infrared bands about two-thirds fall in the AGN region for the Lacy et al criteria, while around a third do so for the Stern et al and the Messias et al criteria.…”