A new infrared color criterion for the selection of 0<z<7 AGN: application to deep fields and implications for JWST surveys

TL;DR

This paper develops new infrared color criteria using JWST wavelength ranges to improve the selection of AGN across a wide redshift range, aiming for higher completeness and lower contamination in deep field surveys.

Contribution

It introduces new IR color diagnostics and criteria (KI and KIM) optimized for JWST data to enhance AGN detection efficiency from low to high redshifts.

Findings

KI achieves 50-60% completeness with 50-90% success rate.

KIM can select AGN up to z~7 with 30-40% completeness.

KI and KIM are robust against flux errors and ready for immediate application.

Abstract

[Abridged] It is widely accepted that observations at mid-infrared (mid-IR) wavelengths enable the selection of galaxies with nuclear activity, which may not be revealed even in the deepest X-ray surveys. In this work new near- and mid-IR color diagnostics are explored, aiming for improved efficiency - better completeness and less contamination - in selecting AGN out to very high redshifts. We restrict our study to the James Webb Space Telescope wavelength range (0.6-27um). The criteria are created based on the predictions by state-of-the-art galaxy and AGN templates covering a wide variety of galaxy properties, and tested against control samples with deep multi-wavelength coverage (ranging from the X-rays to radio frequencies). We show that the colors Ks-[4.5], [4.5]-[8.0], and [8.0]-[24] are ideal as AGN/non-AGN diagnostics at, respectively, z<~1, 1<~z<~2.5, and z>~2.5-3. However, when the source redshift is unknown, these colors should be combined. We thus develop an improved IR criterion (using Ks and IRAC bands, KI) as a new alternative at z<~2.5. KI does not show improved completeness (50-60% overall) in comparison to commonly used IRAC-based AGN criteria, but is less affected by non-AGN contamination (revealing a >50-90% level of successful AGN selection). We also propose KIM (using Ks, IRAC, and MIPS-24um bands, KIM), which aims to select AGN hosts from local distances to as far back as the end of reionization (0<z<~7) with reduced non-AGN contamination. However, the necessary testing-constraints and the small control-sample sizes prevent the confirmation of its improved efficiency at z<~2.5. Overall, KIM shows a ~30-40% completeness and a >70-90% level of successful AGN selection. KI and KIM are built to be reliable against a ~10-20% error in flux, are based on existing filters, and are suitable for immediate use.