A New Infrared Color Criterion for the Selection of 0 < z < 7 AGNs: Application to Deep Fields and Implications for JWST Surveys

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. Many mid-IR color-color criteria have been explored to accomplish this goal and tested thoroughly in the literature. Besides missing many low-luminosity active galactic nuclei (AGNs), one of the main conclusions is that, with increasing redshift, the contamination by non-active galaxies becomes significant (especially at z >~ 2.5). This is problematic for the study of the AGN phenomenon in the early universe, the main goal of many of the current and future deep extragalactic surveys. In this work new near- and mid-IR color diagnostics are explored, aiming for improved efficiency—better completeness and less contamination—in selecting AGNs out to very high redshifts. We restrict our study to the James Webb Space Telescope wavelength range (0.6-27 μm). 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 K_s - [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 K_s 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 Infrared Array Camera (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 K_s , IRAC, and MIPS 24 μm 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.