A 2.5-5 {\mu}m Spectroscopic Study of Hard X-ray Selected AGNs using AKARI InfraRed Camera

TL;DR

This study uses 2.5-5 μm spectroscopy from AKARI to analyze the circumnuclear environment of hard X-ray selected AGNs, finding no significant difference in star formation activity between different AGN types.

Contribution

First application of AKARI 2.5-5 μm spectroscopy to a hard X-ray selected AGN sample, providing insights into star formation and dust features across AGN types.

Findings

No significant difference in PAH emission between AGN types.

PAH luminosity normalized by black hole mass is independent of X-ray column density.

Spectroscopy reveals key dust and molecular features in AGN environments.

Abstract

We present results of the 2.5-5 {\mu}m spectroscopy of a sample of hard X-ray selected active galactic nuclei (AGNs) using the grism mode of the InfraRed Camera (IRC) on board the infrared astronomical satellite AKARI. The sample is selected from the 9-month Swift/BAT survey in the 14-195 keV band, which provides a fair sample of AGNs including highly absorbed ones. The 2.5-5 {\mu}m spectroscopy provide a strong diagnostic tool for the circumnuclear environment of AGNs through the continuum shapes and emission/absorption features such as the 3.3 {\mu}m polycyclic aromatic hydrocarbon (PAH) emission and the broad 3.1 {\mu}m H2O ice, 3.4 {\mu}m bare carbonaceous dust, 4.26 {\mu}m CO2 and 4.67 {\mu}m CO absorptions. As our first step, we use the 3.3 {\mu}m PAH emission as a proxy for the star-formation activity and searched for possible difference of star-formation activity between type 1 (unabsorbed) and type 2 (absorbed) AGNs. We found no significant dependence of the 3.3 {\mu}m PAH line luminosity, normalized by the black hole mass, on optical AGNs types or the X-ray measured column densities.