Near-identical star formation rate densities from Hα and FUV at redshift zero

Abstract

For the first time both Hα and far-ultraviolet (FUV) observations from an Hɪ-selected sample are used to determine the dust-corrected star formation rate density (SFRD: ṗ) in the local Universe. Applying the two star formation rate indicators on 294 local galaxies, we determine log(ṗ Hα) = -1.168 +0.13 -0.05 [M⊙yr^-1 Mpc^-3] and log(ṗ FUV) = -1.171 +0.12 -0.13 [M⊙yr^-1 Mpc^-3]. These values are derived from scaling Hα and FUV observations to the Hɪ mass function. Galaxies were selected to uniformly sample the full Hɪ mass (MHɪ) range of the Hɪ Parkes All-Sky Survey (MHɪ ~ 10^7 to ~10^10.7 M⊙). The approach leads to relatively larger sampling of dwarf galaxies compared to optically selected surveys. The low Hɪ mass, low luminosity, and low surface brightness galaxy populations have, on average, lower Hα/FUV flux ratios than the remaining galaxy populations, consistent with the earlier results of Meurer. The near-identical Hα- and FUV-derived SFRD values arise with the low Hα/FUV flux ratios of some galaxies being offset by enhanced Hα from the brightest and high mass galaxy populations. Our findings confirm the necessity to fully sample the Hɪ mass range for a complete census of local star formation to include lower stellar mass galaxies which dominate the local Universe.