Infrared-detected AGNs in the local Universe

Abstract

Context. Spitzer/IRAC color selection is a promising technique to identify hot accreting nuclei, that is, active galactic nuclei (AGN), in galaxies. We demonstrate this using a small sample of SAURON galaxies and then explore this technique further. Aims. The goal of this study is to find a simple and efficient way to reveal optically obscured nuclear accretion in (nearby) galaxies. Methods. We applied an infrared selection method to the Spitzer Survey of Stellar Structures in Galaxies (S4G) sample of more than 2500 galaxies, together with its extension sample of more than 400 galaxies. We used the Spitzer colors to find galaxies in the S4G survey containing a hot core, suggesting the presence of a strong AGN, and we studied the detection fraction as a function of morphological type. We tested this infrared color selection method by examining the radio properties of the galaxies via the VLA NVSS and FIRST surveys. Results. Using the radio data, we demonstrate that galaxies displaying hot mid-infrared nuclei stand out as (candidate) active galaxies. We reproduce these results when using colors from the lower spatial resolution WISE mission instead of Spitzer. Hence multiband infrared imaging represents a useful tool to uncover optically obscured nuclear activity in galaxies.