Tests of star formation metrics in the low metallicity galaxy NGC 5253 using ALMA observations of H30 line emission

We use Atacama Large Millimeter/submillimeter Array (ALMA) observations of H30 alpha (231.90 GHz) emission from the low-metallicity dwarf galaxy NGC5253 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 +/- 0.3) x 10(52) s(-1) and an SFR of 0.087 +/- 0.013M circle dot 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 a previously published version of the Ha flux that was extinction corrected using Pa alpha and Pa beta lines was 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 Ha 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 SFRs lower than the H30 alpha value, although the SFRs from the 70 mu m and H30 alpha data were within 1 sigma-2 sigma of each other. While further analysis on a broader range of galaxies is needed, these results are instructive of the best and worst methods to use when measuring SFR in low-metallicity dwarf galaxies like NGC5253.