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

TL;DR

The paper systematically quantifies how aromatic and aliphatic hydrocarbon emissions evolve in nearby galaxies by analyzing AKARI/IRC near-IR spectra around 3.3 µm and 3.4–3.6 µm. It uses a two-tier spectral fitting approach (full-range and local-range) plus DustEM-based SED fitting to extract $L_ ext{aromatic}$, $L_ ext{aliphatic}$, and related luminosities, then investigates how $L_ ext{aliphatic}/L_ ext{aromatic}$ varies with $L_ ext{IR}$, Br$ ext{α}$ luminosity, and continuum colors. The study finds $L_ ext{aliphatic}/L_ ext{aromatic}$ spans $0.01$–$1$, generally declines with increasing $L_ ext{IR}$ and UV hardness, and correlates with the 4 µm continuum color, indicating non-processed PAHs in UV-quiet regions and blending-induced suppression in ULIRGs with obscured nuclei. These results imply distinct hydrocarbon dust processing regimes across (sub-)IRGs, LIRGs, and ULIRGs, with implications for the evolution of interstellar organic matter in galaxies.

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 um and 3.4-3.6 um observed with AKARI/IRC. We analyzed 243 near-IR spectra of 240 star-forming (U)LIRGs (the total IR luminosity, $L_\rm{IR}>10^{11}\ L_\odot$), 119 spectra of 105 star-forming IRGs ($10^{10}\ L_\odot<L_\rm{IR}<10^{11}\ L_\odot$), and 94 spectra of 65 sub-IRGs ($L_\rm{IR}<10^{10}\ L_\odot$), in addition to 232 spectra of 36 Galactic HII 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 HI recombination line Br$\rmα$. The result indicates that the luminosity ratios of the aliphatic to the aromatic hydrocarbons ($L_\rm{ali}/L_\rm{aro}$) in the sample galaxies show considerably large variations, compared to those in the Galactic HII regions, $L_\rm{ali}/L_\rm{aro}$ systematically decreasing with $L_\rm{IR}$ and $L_\rm{Brα}$. We find that (sub-)IRGs with continuum colors bluer at 4 um tend to have higher $L_\rm{ali}/L_\rm{aro}$, which is likely to reflect the intrinsic nature of PAHs outside the HII region where the PAHs remain non-processed by strong UV radiation fields. We also find that some ULIRGs with continuum colors redder at 4 um show extremely low $L_\rm{ali}/L_\rm{aro}$, 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.

Figures (13)

• Figure 1: Comparison of $L_\mathrm{IR}$ and redshift, $z$, for the AKARI sample galaxies in this study (red circles), those in kondo2024 (green circles), and the JWST FRESCO sample galaxies in lyu2025 (blue circles), where open marks represent galaxies with $L_\mathrm{IR}$ upper limits. Alt text: One scatter plot.
• Figure 2: Example spectrum showing an unusually broad feature over the wavelength range of 3.1--3.2 µ m. The black, red, and green lines represent the near-IR continuum, the 3.3 µ m aromatic feature, and the 3.4--3.6 µ m aliphatic feature, respectively. Alt text: One example spectrum observed with AKARI/IRC.
• Figure 3: Examples of the near-IR spectral fitting using the local-range model. The spectral model is composed of the near-IR continuum (black line), aromatic hydrocarbon emission feature (red line), aliphatic hydrocarbon emission features (green lines), and H .8 .8 I Pf$\mathrm{\delta}$ recombination line (blue line). The hydrocarbon features and Pf$\mathrm{\delta}$ recombination line are attenuated by the $\mathrm{H_2O}$ ice absorption. Alt text: Twelve panels of examples for near-infrared spectral fitting.
• Figure 4: Examples of the SED fitting to the same galaxies as shown in figure \ref{['example_nir_fitting']}. The SED model is composed of stellar continuum (blue solid line), hot dust continuum (red solid line), ionized PAHs (purple dashed line), neutral PAHs (blue dashed line), small amorphous carbon (green dashed line), large amorphous carbon (yellow dashed line), and amorphous silicate dust (red dashed line). Alt text: Twelve panels of examples for spectral energy distribution fitting.
• Figure 5: Scatter plots between (a) $L_\mathrm{aliphatic}$ and $L_\mathrm{aromatic}$, (b) $L_\mathrm{Br\alpha}$ and $L_\mathrm{aromatic}$, and (c) $L_\mathrm{Br\alpha}$ and $L_\mathrm{aliphatic}$ for all the sample galaxies observed with $S/N>$ 3 for the detection of the aromatic feature at 3.3 µ m and with $S/N>$ 1 for the detection of the aliphatic features at 3.4--3.6 µ m and the H .8 .8 I Br$\mathrm{\alpha}$ recombination line for a display purpose: sub-IRGs (black circles), IRGs (red circles), LIRGs (green circles), and ULIRGs (blue circles). The open marks correspond to the results obtained for off-center regions of a galaxy. Alt text: Three scatter plots.