The Calibration of Polycyclic Aromatic Hydrocarbon Dust Emission as a Star Formation Rate Indicator in the AKARI NEP Survey

Abstract

Polycyclic aromatic hydrocarbon (PAH) dust emission has been proposed as an effective extinction-independent star formation rate (SFR) indicator in the mid-infrared (MIR), but this may depend on conditions in the interstellar medium. The coverage of the AKARI/Infrared Camera (IRC) allows us to study the effects of metallicity, starburst intensity, and active galactic nuclei on PAH emission in galaxies with $f_{\hat{I}}½(L18W)\lesssim 19$ AB mag. Observations include follow-up, rest-frame optical spectra of 443 galaxies within the AKARI North Ecliptic Pole survey that have IRC detections from 7-24 $\hat{I}¼$m. We use optical emission line diagnostics to infer SFR based on H$\hat{I}\pm$ and [O II]$\hat{I}»\hat{I}»3726,3729$ emission line luminosities. The PAH 6.2 $\hat{I}¼$m and PAH 7.7 $\hat{I}¼$m luminosities ($L(PAH6.2\hat{I}¼m)$ and $L(PAH7.7\hat{I}¼m)$, respectively) derived using multi-wavelength model fits are consistent with those derived from slitless spectroscopy within 0.2 dex. $L(PAH6.2\hat{I}¼m)$ and $L(PAH7.7\hat{I}¼m)$ correlate linearly with the 24 $\hat{I}¼$m-dust corrected H$\hat{I}\pm$ luminosity only for normal, star-forming ``main-sequence" galaxies. Assuming multi-linear correlations, we quantify the additional dependencies on metallicity and starburst intensity, which we use to correct our PAH SFR calibrations at $0<z<1.2$ for the first time. We derive the cosmic star formation rate density (SFRD) per comoving volume from $0.15\lesssim z\lesssim 1$. The PAH SFRD is consistent with that of the far-infrared and reaches an order of magnitude higher than that of uncorrected UV observations at $z\sim 1$. Starburst galaxies contribute $\gtrsim 0.7$ of the total SFRD at $z\sim 1$ compared to main-sequence galaxies.

Accepted for publication in The Astrophysical Journal. 50 pages, 27 figures, 9 tables