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