The Calibration of Short Wavelength Polycyclic Aromatic Hydrocarbon Emission as Star Formation Rate Indicators with JWST

TL;DR

This study calibrates PAH emission features as star formation rate indicators using JWST imaging across nearby galaxies, revealing metallicity-dependent variations and the influence of older stars on PAH excitation.

Contribution

It provides the first detailed calibration of PAH features as SFR indicators at 40 pc scales with JWST, highlighting metallicity effects and PAH size distribution shifts.

Findings

PAH luminosities correlate tightly with SFR in near solar metallicity environments.

PAH features are weaker and more scattered in metal-poor galaxy NGC 4449.

The 3.3/7.7 μm PAH ratio varies with metallicity and SFR, indicating size distribution changes.

Abstract

We use JWST/NIRCam and MIRI imaging acquired by the Feedback in Emerging extrAgalactic Star clusTers (FEAST) program along with archival HST imaging to map ionized gas (Pa$\alpha$, Br$\alpha$, and H$\alpha$) and Polycyclic Aromatic Hydrocarbon (PAH) emission (3.3 and 7.7 $\mu$m) across a sample of four nearby galaxies (NGC 5194, 5236, 628, and 4449). These maps are utilized to calibrate the PAH features as star formation rate (SFR) indicators in 40 pc size regions around massive emerging young star clusters (eYSCs). We find a tight, sub-linear (power-law exponent, $\alpha{\,}{\sim}{\,}0.8$) relation between the PAH luminosities (3.3 and 7.7 $\mu$m) and SFR (extinction corrected Pa$\alpha$) in near solar metallicity environments. PAH destruction in more intense ionizing environments and/or variations in the age of our sources may drive the deviation from a linear relation. In the metal-poor environment of NGC 4449 (${\sim}$1/3 Z$_{\odot}$), we see substantial deficits in the PAH feature strengths at fixed SFR and significantly higher scatter in the PAH-SFR relations. We determine that the 3.3/7.7 $\mu$m PAH luminosity ratio increases towards lower metallicity environments. This is interpreted as a result of a shift in the size distribution towards smaller PAHs at lower metallicities, possibly due to inhibited grain growth. Focusing on the regions in NGC 4449, we observe a decreasing 3.3/7.7 $\mu$m ratio towards higher SFR, which could indicate that small PAHs are preferentially destroyed relative to larger PAHs in significantly sub-solar metallicity conditions. We estimate that ${\sim}$2/3 of the PAH emission in typical local star-forming galaxies is excited by older stars and unrelated to recent ($<$10 Myr) star formation.