A systematic study on the properties of aromatic and aliphatic hydrocarbon dust in active galactic nuclei with AKARI near-infrared spectroscopy

¹ Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602, Japan
² Institute of Liberal Arts and Sciences, Tokushima University, 1-1 Minami-Jyosanjima, Tokushima-shi, Tokushima 770-8502, Japan
³ Institute of Space and Astronomical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara Kanagawa 252-5210, Japan

^★ Corresponding author: r.katayama@u.phys.nagoya-u.ac.jp

Received: 5 May 2025
Accepted: 11 September 2025

Abstract

Recent near- and mid-infrared (IR) observations have revealed the existence of appreciable amounts of aromatic and aliphatic hydrocarbon dust in the harsh environments of active galactic nuclei (AGNs), the origins of which are still under discussion. In this paper, we analyze the near-IR spectra of AGNs obtained with AKARI in order to systematically study the properties of the aromatic and aliphatic hydrocarbon dust affected by AGN activity. We performed spectral fitting and spectral energy distribution fitting for our sample of 102 AGNs to obtain the fluxes of the aromatic and aliphatic spectral features, the total IR luminosity (L_IR), and the fractional luminosity of AGN components (L_AGN/L_IR). As a result, we find that L_aromatic/L_IR is systematically lower for the AGN sample and especially lower for AGNs with the aliphatic feature seen in the absorption than for star-forming galaxies (SFGs), while L_aliphatic/L_aromatic is systematically higher for the AGN sample than for the SFG sample, increasing with AGN activity indicated by L_AGN/L_IR. In addition, the profiles of the aliphatic emission features of the AGN sample are significantly different from those of the SFG sample in that the AGNs have systematically stronger feature intensities at longer wavelengths. We conclude that both aromatic and aliphatic hydrocarbon dust are likely of circumnuclear origin, suggesting that a significant amount of the aliphatic hydrocarbon dust may come from a new population created through processes such as the shattering of large carbonaceous grains by AGN outflows.