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

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 micron aliphatic C--H stretch, a weak satellite feature accompanying the 3.3 micron 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 micron aromatic and 3.4 micron aliphatic C--H emission. We also compiled their multi-wavelength 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 micron PAH emission luminosity ($L_{3.3}$) as a calibrator of the SFR and found a close agreement with previous studies. We also found that $L_{3.3}/L_{\rm IR}$ and $L_{3.4}/L_{\rm IR}$ exhibit a strong dependence on metallicity, but remain nearly constant above 12+log(O/H)$\sim\,$8.5, where $L_{\rm IR}$ is the total luminosity emitted by dust, and $L_{3.4}$ is the luminosity of the 3.4 micron aliphatic emission. We derived from $L_{3.4}/L_{3.3}$ the PAH aliphatic fractions, defined as the fractions of carbon atoms in aliphatic units, to be in the range of $\sim\,$0.38%--6.8%, with a median fraction of $\sim\,$3.1%. The PAH aliphatic fractions are lower in AGN hosts and show a weak negative correlation with the SFR and $L_{\rm IR}$, suggesting that UV photons in regions with AGN or strong star formation activities may photodissociate the aliphatic structures associated with PAH molecules.