All the PAHs: an AKARI-Spitzer Cross Archival Spectroscopic Survey of Aromatic Emission in Galaxies

TL;DR

This study analyzes a large sample of galaxy spectra to model PAH emissions, especially at 3.3 μm, revealing their relation to star formation and environmental effects, with implications for JWST observations.

Contribution

First comprehensive modeling of all PAH emission bands in galaxy spectra using AKARI and Spitzer data, including the 3.3 μm feature, and establishing its calibration with star formation rate.

Findings

The 3.3 μm PAH contributes about 1.5-3% of total PAH power.

The 3.3 μm PAH emission correlates with star formation rate but varies with luminosity and metallicity.

The 3.3 μm PAH feature remains strong in low metallicity dwarf galaxy II Zw 40.

Abstract

We present a large sample of 2.5-38 $\mu m$ galaxy spectra drawn from a cross-archival comparison in the AKARI-Spitzer Extragalactic Spectral Survey (ASESS), and investigate a subset of 113 star-forming galaxies with prominent polycyclic aromatic hydrocarbon (PAH) emission spanning a wide range of star formation properties. With AKARI's extended 2.5-5 $\mu m$ wavelength coverage, we self-consistently model for the first time all PAH emission bands using a modified version of PAHFIT. We find $L_\mathrm{PAH\,3.3}/L_\mathrm{IR}$ $\sim$ 0.1% and the 3.3 $\mu m$ PAH feature contributes $\sim$1.5-3% to the total PAH power -- somewhat less than earlier dust models have assumed. We establish a calibration between 3.3 $\mu m$ PAH emission and star formation rate, but also find regimes where it loses reliability, including at high luminosity and low metallicity. The 3.4 $\mu m$ aliphatic emission and a broad plateau feature centered at 3.47 $\mu m$ are also modeled. As the shortest wavelength PAH feature, 3.3 $\mu m$ is susceptible to attenuation, leading to a factor of $\sim$ 3 differences in the inferred star formation rate at high obscuration with different assumed attenuation geometries. Surprisingly, $L_\mathrm{PAH\, 3.3} / L_\mathrm{\Sigma\, PAH}$ shows no sign of decline at high luminosities, and the low metallicity dwarf galaxy II Zw 40 exhibits an unusually strong 3.3 $\mu m$ band; both results suggest either that the smallest PAHs are better able to survive under intense radiation fields than presumed, or that PAH emission is shifted to shorter wavelengths in intense and high energy radiation environments. A photometric surrogate for 3.3 $\mu m$ PAH luminosity using JWST/NIRCam is provided and found to be highly reliable at low redshift.