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Peer-reviewedCC BY 4.0The Astrophysical Journal

Aromatics and Aliphatics in Local Star-forming Galaxies as Probed by AKARI

  • Junhao Peng1
  • Xuejuan Yang1,2
  • Aigen Li2
  1. 1Xiangtan University
  2. 2University of Missouri
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Abstract

Polycyclic aromatic hydrocarbon (PAH) molecules are abundant and widespread in galaxies, and their infrared (IR) emission traces star formation. PAH molecules in astronomical environments often have aliphatic contents as revealed by the detection of the 3.4 μ m aliphatic C–H stretch, a weak satellite feature accompanying the 3.3 μ m aromatic C–H stretch. Here we selected 102 local star-forming galaxies from the AKARI archive, including 66 galaxies, each of which hosts an active galactic nucleus (AGN). We analyzed their AKARI near-IR spectra, which exhibit pronounced 3.3 μ m aromatic and 3.4 μ m aliphatic C–H emission. We also compiled their multiwavelength photometric data and performed a decompositional analysis of their spectral energy distributions (SEDs) from the ultraviolet (UV) to the far-IR to derive the star formation rates (SFRs), stellar masses, metallicities, and luminosity of the galaxies. We explored the 3.3 μ m PAH emission luminosity ( L 3.3 ) as a calibrator of the SFR and found a close agreement with previous studies. We also found that L 3.3 / L IR and L 3.4 / L IR exhibit a strong dependence on metallicity but remain nearly constant above 12 + log(O/H) ∼ 8.5, where L IR is the total luminosity emitted by dust and L 3.4 is the luminosity of the 3.4 μ m aliphatic emission. From L 3.4 / L 3.3 we derived the PAH aliphatic fractions, defined as the fractions of carbon atoms in aliphatic units, to be in the range of ∼0.38%–6.8%, with a median fraction of ∼3.1%. The PAH aliphatic fractions are lower in AGN hosts and show a weak negative correlation with the SFR and L IR , suggesting that UV photons in regions with AGN or strong star formation activities may photodissociate the aliphatic structures associated with PAH molecules.

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Cite

BibTeXRIS
BibTeX
@article{peng2026aromatics,
  title = {{Aromatics and Aliphatics in Local Star-forming Galaxies as Probed by AKARI}},
  author = {Junhao Peng and Xuejuan Yang and Aigen Li},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/ae954d},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/ae954d},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Aromatics and Aliphatics in Local Star-forming Galaxies as Probed by AKARI
AU  - Junhao Peng
AU  - Xuejuan Yang
AU  - Aigen Li
PY  - 2026
DA  - 2026/10/15
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/ae954d
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/ae954d
AB  - Polycyclic aromatic hydrocarbon (PAH) molecules are abundant and widespread in galaxies, and their infrared (IR) emission traces star formation. PAH molecules in astronomical environments often have aliphatic contents as revealed by the detection of the 3.4 μ m aliphatic C–H stretch, a weak satellite feature accompanying the 3.3 μ m aromatic C–H stretch. Here we selected 102 local star-forming galaxies from the AKARI archive, including 66 galaxies, each of which hosts an active galactic nucleus (AGN). We analyzed their AKARI near-IR spectra, which exhibit pronounced 3.3 μ m aromatic and 3.4 μ m aliphatic C–H emission. We also compiled their multiwavelength photometric data and performed a decompositional analysis of their spectral energy distributions (SEDs) from the ultraviolet (UV) to the far-IR to derive the star formation rates (SFRs), stellar masses, metallicities, and luminosity of the galaxies. We explored the 3.3 μ m PAH emission luminosity ( L 3.3 ) as a calibrator of the SFR and found a close agreement with previous studies. We also found that L 3.3 / L IR and L 3.4 / L IR exhibit a strong dependence on metallicity but remain nearly constant above 12 + log(O/H) ∼ 8.5, where L IR is the total luminosity emitted by dust and L 3.4 is the luminosity of the 3.4 μ m aliphatic emission. From L 3.4 / L 3.3 we derived the PAH aliphatic fractions, defined as the fractions of carbon atoms in aliphatic units, to be in the range of ∼0.38%–6.8%, with a median fraction of ∼3.1%. The PAH aliphatic fractions are lower in AGN hosts and show a weak negative correlation with the SFR and L IR , suggesting that UV photons in regions with AGN or strong star formation activities may photodissociate the aliphatic structures associated with PAH molecules.
SN  - 0004-637X
SN  - 1538-4357
ER  -