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

Abstract

We present a large sample of 2.5-38 µm galaxy spectra drawn from a cross-archival comparison in the AKARI -Spitzer Extragalactic Spectral Survey (ASESS), and investigate a subset of 113 starforming galaxies with prominent polycyclic aromatic hydrocarbon (PAH) emission spanning a wide range of star formation properties. With AKARI 's extended 2.5-5 µm wavelength coverage, we selfconsistently model for the first time all PAH emission bands using a modified version of Pahfit. We find L PAH 3.3 / L IR ∼ 0.1% and the 3.3 µm PAH feature contributes ∼1.5-3% to the total PAH power -somewhat less than earlier dust models have assumed. We establish a calibration between 3.3 µm PAH emission and star formation rate, but also find regimes where it loses reliability, including at high luminosity and low metallicity. The 3.4 µm aliphatic emission and a broad plateau feature centered at 3.47 µm are also modeled. As the shortest wavelength PAH feature, 3.3 µm is susceptible to attenuation, leading to a factor of ∼ 3 differences in the inferred star formation rate at high obscuration with different assumed attenuation geometries. Surprisingly, L PAH 3.3 /L Σ PAH shows no sign of decline at high luminosities, and the low metallicity dwarf galaxy II Zw 40 exhibits an unusually strong 3.3 µm 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 µm PAH luminosity using JWST /NIRCam is provided and found to be highly reliable at low redshift.

Regulating Star Formation in Nearby Dusty Galaxies: Low Photoelectric Efficiencies in the Most Compact Systems

“…These galaxies have full spectral model fits to the AKARI+Spitzer data, which explicitly includes silicate absorption and 3.05 µm ice feature absorption which are fit independently. Joint fits to IR continuum and line measurements are important to get accurate line fluxes e.g., Smith et al 2007;Lai et al 2020), and we use these new CAFE fits to estimate extinction-corrected 3.3 µm luminosities, L 3.3 , for the full AKARI sample. In general, the measured PAH intensity depends on how the local continuum is estimated.…”