鈴木 仁研

Abstract Interstellar hydrocarbon dust containing aromatic and aliphatic hydrocarbons, like polycyclic aromatic hydrocarbons (PAHs), is believed to be processed by various factors including UV radiation fields and mechanical shocks in the galactic environments. We systematically investigate the processing of hydrocarbon dust, especially the likely causes for the variations of the luminosity ratio of aliphatic to aromatic hydrocarbon emission features, using the near-infrared (IR) spectral features at wavelengths 3.3 μm and 3.4–3.6 μm observed with AKARI/IRC. We analyzed 243 near-IR spectra of 240 star-forming (ultra-)luminous IR galaxies (total IR luminosity, $L_\mathrm{IR}>10^{11}\, {L_\odot }$), 119 spectra of 105 star-forming IR galaxies ($10^{10}\, {L_\odot }< L_\mathrm{IR}< 10^{11}\, {L_\odot }$), and 94 spectra of 65 sub-IR galaxies ($L_\mathrm{IR}< 10^{10}\, {L_\odot }$), in addition to 232 spectra of 36 Galactic H ii regions as a reference sample. We performed near-IR spectral model fitting to estimate the luminosities of the aromatic and aliphatic hydrocarbon features and the H i recombination line Br${\alpha }$. The result indicates that the luminosity ratios of the aliphatic to the aromatic hydrocarbons ($L_\mathrm{aliphatic}/L_\mathrm{aromatic}$) in the sample galaxies show considerably large variations, compared to those in the Galactic H ii regions, $L_\mathrm{aliphatic}/L_\mathrm{aromatic}$ systematically decreasing with $L_\mathrm{IR}$ and $L_\mathrm{Br\alpha }$. We find that (sub-)IR galaxies with continuum colors bluer at 4 μm tend to have higher $L_\mathrm{aliphatic}/L_\mathrm{aromatic}$, which is likely to reflect the intrinsic nature of PAHs outside the H ii region where the PAHs remain non-processed by strong UV radiation fields. We also find that some ultra-luminous IR galaxies with continuum colors redder at 4 μm show extremely low $L_\mathrm{aliphatic}/L_\mathrm{aromatic}$, which is likely to be caused by blending aliphatic emission and absorption features due to the presence of an obscured galactic nucleus in merger systems.

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.

Centaurus A (Cen A) is the nearest galaxy hosting an active galactic nucleus (AGN), which produces powerful radio and X-ray jets extending to hundreds of kiloparsecs from the center. At 15 kpc northeast (NE) and 12 kpc southwest (SW) in the halo along the jet from the nucleus of Cen A, dust clouds accompanying the Hα emission are detected. For both NE and SW clouds, past studies suggested that star formation may have been induced through interactions between the AGN jet and the surrounding intergalactic media. For these clouds, we performed dust model fitting of infrared (IR) spectral energy distributions (SEDs) created from the archival data of WISE, Spitzer, and Herschel. Then we compare the IR emission properties of the dust clouds with the far-ultraviolet (UV) emission using the archival data of GALEX/FUV. As a result, we find that the interstellar radiation field intensity G₀ (and thus the dust temperature) in the NE cloud suggests star formation activity, while that in the SW cloud does not. The local far-UV intensity and G₀ in the NE region are significantly larger than those expected for the far-UV radiation originating from the central region of Cen A and its dust-scattered component, respectively. In contrast, the local far-UV intensity and G₀ in the SW region are compatible with them. The polycyclic aromatic hydrocarbon (PAH) emission is detected for both NE and SW clouds. The mass abundance ratios of PAH to dust are similar for both clouds and significantly lower than that in the central region of Cen A. We suggest that the dust clouds and the PAHs in the clouds are associated with the broken ring-like structure of H I gas which is thought to be a remnant of the past gas-rich merger and that shocks by the jet responsible for the middle lobe on the north side may have triggered the star formation in the NE cloud.