Tests of star formation metrics in the low metallicity galaxy NGC 5253
  using ALMA observations of H30$\alpha$ line emission
  We use Atacama Large Millimeter/submillimeter Array (ALMA) observations of
H30$\alpha$ (231.90 GHz) emission from the low metallicity dwarf galaxy NGC
5253 to measure the star formation rate (SFR) within the galaxy and to test the
reliability of SFRs derived from other commonly-used metrics. The H30$\alpha$
emission, which originates mainly from the central starburst, yields a
photoionizing photon production rate of (1.9$\pm$0.3)$\times$10$^{52}$ s$^{-1}$
and an SFR of 0.087$\pm$0.013 M$_\odot$ yr$^{-1}$ based on conversions that
account for the low metallicity of the galaxy and for stellar rotation. Among
the other star formation metrics we examined, the SFR calculated from the total
infrared flux was statistically equivalent to the values from the H30$\alpha$
data. The SFR based on previously-published versions of the H$\alpha$ flux that
were extinction corrected using Pa$\alpha$ and Pa$\beta$ lines were lower than
but also statistically similar to the H30$\alpha$ value. The mid-infrared (22
$\mu$m) flux density and the composite star formation tracer based on H$\alpha$
and mid-infrared emission give SFRs that were significantly higher because the
dust emission appears unusually hot compared to typical spiral galaxies.
Conversely, the 70 and 160 $\mu$m flux densities yielded SFR lower than the
H30$\alpha$ value, although the SFRs from the 70 $\mu$m and H30$\alpha$ data
were within 1-2$\sigma$ of each other. While further analysis on a broader
range of galaxies are needed, these results are instructive of the best and
worst methods to use when measuring SFR in low metallicity dwarf galaxies like
NGC 5253.
