The hunt for red AGN: a new infrared diagnostic

TL;DR

This paper presents a new infrared diagnostic method that effectively distinguishes between stellar and nuclear galaxy emissions, enabling better identification of AGN up to redshift 2.5 using existing and future infrared survey data.

Contribution

The paper introduces a novel broadband infrared diagnostic that combines silicate absorption and PAH features for galaxy classification, applicable to multiple infrared surveys.

Findings

70% of X-ray and optically detected AGN dominate mid-infrared emission.

The diagnostic distinguishes galaxy types up to redshift 2.5.

AGN contribute less than 30% to the total mid-infrared extragalactic emission.

Abstract

We introduce a new infrared diagnostic to separate galaxies on the basis of their dominant infrared emission: stellar or nuclear. The main novelty with respect to existing diagnostics, is the usage of a broad band encompassing at the same time the 9.7micron Silicate absorption feature and one of the adjacent broad PAH (polycyclic aromatic hydrocarbon) features. This provides a robust estimate of the near- to mid-infrared continuum slope and enables a clear distinction among different classes of galaxies up to a redshift z=2.5. The diagnostic can be applied to a wealth of archival data from the ISO, Spitzer, and Akari surveys as well as future JWST surveys. Based on data in the GOODS, Lockman Hole, and North Ecliptic Pole (NEP) fields, we find out that approximately 70% active galactic nuclei detected with X-ray and optical spectroscopy dominate the total mid-infrared emission. Finally, we estimate that AGN contribute less than 30% of the mid-infrared extragalactic integrated emission.