The 3.3 micron PAH Emission as a Star Formation Rate Indicator

TL;DR

This study investigates the potential of the 3.3 micron PAH emission feature as a star formation rate indicator by analyzing spectra from galaxies, finding a correlation with infrared luminosity but with limitations in ultra-luminous infrared galaxies.

Contribution

First detection and calibration of the 3.3 micron PAH feature as a star formation rate indicator using new and literature data, highlighting its limitations in ULIRGs.

Findings

3.3 micron PAH emission detected in 3 of 20 galaxies

Correlation between PAH luminosity and infrared luminosity with large scatter

Correlation breaks down in ULIRGs, limiting its use as a proxy

Abstract

Polycyclic Aromatic Hydrocarbon (PAH) emission features dominate the mid-infrared spectra of star-forming galaxies and can be useful to calibrate star formation rates and diagnose ionized states of grains. However, the PAH 3.3 micron feature has not been studied as much as other PAH features since it is weaker than others and resides outside of Spitzer capability. In order to detect and calibrate the 3.3 micron PAH emission and investigate its potential as a star formation rate indicator, we carried out an AKARI mission program, AKARI mJy Unbiased Survey of Extragalactic Survey (AMUSES) and compare its sample with various literature samples. We obtained 2 ~5 micron low resolution spectra of 20 flux-limited galaxies with mixed SED classes, which yields the detection of the 3.3 micron PAH emission from three out of 20 galaxies. For the combined sample of AMUSES and literature samples, the 3.3 micron PAH luminosities correlate with the infrared luminosities of star-forming galaxies, albeit with a large scatter (1.5 dex). The correlation appears to break down at the domain of ultra-luminous infrared galaxies (ULIRGs), and the power of the 3.3 micron PAH luminosity as a proxy for the infrared luminosity is hampered at log[L(PAH3.3)/(erg/sec)] > -42.0. Possible origins for this deviation in the correlation are discussed, including contribution from AGN and strongly obscured YSOs, and the destruction of PAH molecules in ULIRGs.