Infrared-detected AGNs in the Local Universe

TL;DR

This study demonstrates that infrared color selection using Spitzer and WISE data effectively identifies optically obscured active galactic nuclei (AGNs) in nearby galaxies, validated by radio observations, offering a simple method for detecting hidden nuclear activity.

Contribution

The paper introduces and validates a straightforward infrared color selection technique to detect obscured AGNs in a large galaxy sample, expanding the tools for studying nuclear activity.

Findings

Infrared colors correlate with radio indicators of AGN activity.

The method successfully identifies obscured AGNs in the S4G galaxy sample.

WISE colors reproduce Spitzer-based AGN detections.

Abstract

Spitzer/IRAC color selection is a promising technique to identify hot accreting nuclei, that is to say AGN, in galaxies. We demonstrate this using a small sample of SAURON galaxies, and explore this further. The goal of this study is to find a simple and efficient way to reveal optically obscured nuclear accretion in (nearby) galaxies. We apply 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 use the Spitzer colors to find galaxies in the S$^{4}$G survey containing a hot core, suggesting the presence of a strong AGN, and study the detection fraction as a function of morphological type. We test this infrared color selection method by examining the radio properties of the galaxies, using the VLA NVSS and FIRST surveys. Using the radio data, we demonstrate that galaxies displaying hot mid-infrared nuclei stand out as being (candidate) active galaxies. When using, instead of Spitzer, colors from the lower spatial resolution WISE mission, we reproduce these results. Hence multi-band infrared imaging represents a useful tool to uncover optically obscured nuclear activity in galaxies.