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

TL;DR

The study leverages AKARI near-infrared spectroscopy to systematically quantify aromatic PAH and aliphatic hydrocarbon dust in AGN environments, evaluating how AGN activity shapes their signatures. Through spectral fitting and DustEM-based SED modeling for 102 AGNs, it derives fluxes for the 3.3 μm aromatic feature and 3.4–3.6 μm aliphatic features, along with $L_{\rm IR}$ and $L_{\rm AGN}$. The key findings show that $L_{\rm aromatic}/L_{\rm IR}$ is generally suppressed in AGNs, especially when a-C:H absorption is present, while $L_{\rm aliphatic}/L_{\rm aromatic}$ is elevated and rises with AGN activity, with aliphatic emission profiles indicating a circumnuclear origin and possibly a new dust population produced by AGN outflows via grain-shattering. These results imply active processing of hydrocarbon dust in the circumnuclear regions and highlight distinct dust growth and destruction pathways in AGN-dominated environments, contributing to our understanding of dust lifecycle in galaxies hosting active nuclei.

Abstract

Recent near- and mid-infrared (IR) observations reveal 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 to systematically study the properties of the aromatic and aliphatic hydrocarbon dust a ected by the AGN activity. We perform the spectral fitting and the 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, especially much 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 the AGN activity indicated by L_AGN/L_IR. In addition, the profiles of the aliphatic emission features of the AGN sample are significantly di erent from those of the SFG sample in that the AGNs have the feature intensities systematically stronger at longer wavelengths. We conclude that both aromatic and aliphatic hydrocarbon dust are likely of circumnuclear origins, suggesting that a significant amount of the aliphatic hydrocarbon dust may come from a new population created through processes such as shattering of large carbonaceous grains by AGN outflows.

Figures (14)

• Figure 1: Examples of the spectral fitting results for the AKARI/IRC rest frame 2.5--4.2 $\mu$m spectra of AGN-dominated and AGN-SF-composite galaxies. The top two rows show examples of the cases where the aliphatic hydrocarbon features are detected in the emission, while the bottom two rows show those detected in the absorption. A black line shows the total model spectrum, while red and blue lines correspond to the aromatic and aliphatic hydrocarbon features, respectively.
• Figure 2: Examples of the SED fitting results for the same galaxies as in Fig. \ref{['fig:specfit_sample']}. A black line shows the total model spectrum, while a red line corresponds to the continuum emitted by hot dust. Grey lines show the stellar continuum, neutral and ionized PAHs, the continua emitted by warm dust components, and the continua emitted by SamC, LamC, and aSil, respectively, from short to long wavelengths. Black circles and blue squares represent the fluxes observed in the respective bands and predicted by the model SED, respectively.
• Figure 3: Same as Fig. \ref{['fig:sed_sample']} but the SEDs were fit with a model containing AGN template spectra (see text for details).
• Figure 4: Scatter plot of $L_{\rm{aromatic}}/L_{\rm{IR}}$ as a function of $L_{\rm{IR}}$ for the AGN-dominated (red circles) and AGN-SF-composite galaxies (red triangles). The gray circles show the relationship for the SFG sample.
• Figure 5: (a) Same as Fig. \ref{['fig:lir_aro']} but color coded separately for the aliphatic feature detected in the emission (red) and in the absorption (blue). (b) Scatter plot of $\tau_{\rm{3.41}}$ as a function of $L_{\rm{IR}}$ for the AGN sample.