Tsubasa KONDO, Hidehiro KANEDA, Shinki OYABU, Takuma KOKUSHO, Toyoaki SUZUKI, Risako KATAYAMA, Eiko KOZAKI, Itsuka YACHI, Keita YOSHIDA, Shohei ONO
7 min
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.
Alex: Okay, so that full sweep cleans the data pool. But you mentioned the underlying glow can be tricky to nail down perfectly for the hydrocarbons themselves?
Sam: Right—they follow up with a zoomed-in fit just around the hydrocarbon bumps, using a simpler sloping line for the background glow, or adding a hot-dust curve if needed for brighter galaxies. This local-range approach gives more reliable brightnesses for the chains and rings. The paper suggests these luminosities show the chain-to-ring ratio dropping steadily with overall infrared brightness, and it links to Brα strength too, pointing to ultraviolet radiation as the processor.
Alex: Huh. So stronger radiation traced by Brα chews up chains faster.
Sam: Precisely. This methodical separation reveals the dust's evolution story, from preserved chains in mild environments to aromatic dominance under intense processing. The ratio spans from about 0.01 to 1 across galaxies, dropping notably from medians of around 0.43 in the dimmest ones to 0.15 in the brightest.
Alex: So for quieter galaxies, why do they hold onto more chains—is it just weaker radiation overall?
Sam: In those dimmer ones, more light comes from older stars than young hot ones. That means softer ultraviolet fields outside intense star pockets, letting fragile chains survive on polycyclic cores. Stacked spectra of the bluest, non-active examples reveal pristine chain features, like unweathered originals from galactic outskirts.
Alex: And for the bright mergers with low ratios, what's heating that dust so much?
Sam: Hot dust components, traced by their infrared output fraction, redden the four-micron light and link to the lowest ratios. Buried starbursts in mergers or compact obscured nuclei could power this, scattering chains' signals amid thick dust—confirming violent environments strip aliphatics fastest.
Alex: So overall, quieter galaxies keep more of those fragile chains because their radiation is milder, while mergers grind them down fastest through intense activity and dust hiding.
Sam: That's the core pattern. The paper concludes the ratio changes mainly come from shifts in chain brightness, not ring brightness—higher in less active galaxies with dust untouched by strong radiation outside star-forming pockets, and much lower in merger-driven ultra-bright ones where thick dust around nuclei blends chain glow with absorption dips.
Alex: Blends with absorption? Like the chains get masked in those buried centers?
Sam: Yes—in about a third of the ultra-bright sample, mid-infrared data flags heavily obscured nuclei from mergers. There, chain features appear in absorption at 3.4 microns, mixing with emission to make the glow look weaker overall. This underestimates the chain output, pushing ratios especially low.
Alex: That paints a clear evolution story. But with AKARI blending whole galaxies into one spectrum, doesn't that mix outer calm zones with inner chaos?
Sam: A fair point—the telescope's resolution averages nuclear hearts with outer arms, so ratios reflect galaxy-wide averages, not pinpointed processing. The paper calls for sharper views, like from JWST, to separate regions and model both glow and absorption properly.
Alex: Makes sense—averaging hides details. So sharper tools could map this dust change right to star formation spots over cosmic history.
Sam: Precisely. JWST's near-infrared spectrograph should deliver resolved maps of hydrocarbon processing, tying dust shifts to feedback from stars across time. This work lays a solid baseline for that.
Alex: A meaningful step in understanding how galaxies reshape their own dust through turmoil. Thanks, Sam—that's our look at hydrocarbon evolution in infrared galaxies.
Sam: My pleasure, Alex.