A systematic study on the aromatic and aliphatic hydrocarbon emission features of nearby galaxies using AKARI near-IR spectra

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.