Properties of IR-selected active galactic nuclei

Abstract

Context. Active galactic nuclei (AGNs) of galaxies play an important role in the life and evolution of galaxies through the impact they exert on certain properties and on the evolutionary path of galaxies. It is well known that infrared (IR) emission is useful for selecting galaxies with AGNs, although it has been observed that there is contamination by star-forming galaxies. Aims. We investigate the properties of galaxies that host AGNs that are identified at mid- (MIR) and near-IR wavelengths. The sample of AGNs selected at IR wavelengths was confirmed using optical spectroscopy and X-ray photometry. We study the near-UV, optical, near-IR and MIR properties, as well as the [O III] λ5007 luminosity, black hole mass, and morphology properties of optical and IR colour-selected AGNs. Methods. We selected AGN candidates using two MIR colour selection techniques: a power-law emission method, and a combination of MIR and near-IR selection techniques. We confirmed the AGN selection with two line diagnostic diagrams that use the ratio [O III]/Hβ and the emission line width σ[O III] (kinematics–excitation diagram, KEx) and the host galaxy stellar mass (mass–excitation diagram, MEx), as well as X-ray photometry. Results. According to the diagnostic diagrams, the methods with the greatest success in selecting AGNs are those that use a combination of a mid- and near-IR selection technique and a power-law emission. The method that uses a combination of MIR and near-IR observations selects a large number of AGNs and is reasonably efficient in the success rate (61%) and total number of AGNs recovered. We also find that the KEx method presents contamination of star-forming galaxies within the AGN selection box. According to morphological studies based on the Sérsic index, AGN samples have higher percentages of galaxy morphologies with bulge+disk components than galaxies without AGNs.