Brackett-$\gamma$ as a Gold-standard Test of Star Formation Rates Derived from SED Fitting

TL;DR

This study validates Brackett-$\gamma$ as a reliable, dust-insensitive indicator for star formation rates, using SED fitting and Bayesian inference in local galaxies, with implications for JWST observations.

Contribution

It demonstrates that Br$\gamma$ line predictions from SED fitting are accurate and robust, providing a new standard for SFR measurement unaffected by dust model uncertainties.

Findings

Br$\gamma$ luminosities predicted with small offsets (~0.05 dex)

Predicted equivalent widths agree within ~0.2 dex

SFRs remain accurate despite discrepancies in dust attenuation modeling

Abstract

Using a local reference sample of 21 galaxies, we compare observations of the $\lambda$2.16 $\mu$m Brackett-$\gamma$ (Br$\gamma$) hydrogen recombination line with predictions from the Prospector Bayesian inference framework, which was used to fit the broadband photometry of these systems. This is a clean test of the spectral-energy-distribution-derived star formation rates (SFRs), as dust is expected to be optically thin at this wavelength in nearly all galaxies; thus, the internal conversion of SFR to predicted line luminosity does not depend strongly on the adopted dust model and posterior dust parameters, as is the case for shorter-wavelength lines such as H$\alpha$. We find that Prospector predicts Br$\gamma$ luminosities and equivalent widths with small offsets ($\sim$0.05 dex), and scatter ($\sim$0.2 dex), consistent with measurement uncertainties, though we caution that the derived offset is dependent on the choice of stellar isochrones. We demonstrate that even when the Prospector-derived dust attenuation does not well describe, e.g., H$\alpha$ line properties or observed reddening between H$\alpha$ and Br$\gamma$, the underlying SFRs are accurate, as verified by the dust-free Br$\gamma$ comparison. Finally, we discuss in what ways Br$\gamma$ might be able to help constrain model parameters when treated as an input to the model, and comment on its potential as an accurate monochromatic SFR indicator in the era of JWST multiobject near-IR spectroscopy.