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(abridged) Using a complete sample of ~300 star-forming galaxies within 11Mpc, we evaluate the consistency between star formation rates (SFRs) inferredfrom the far ultraviolet (FUV) non-ionizing continuum and H-alpha nebularemission, assuming standard conversion recipes in which the SFR scales linearlywith luminosity at a given wavelength. Our analysis probes SFRs over 5 ordersof magnitude, down to ultra-low activities on the order of ~0.0001 M_sun/yr.The data are drawn from the 11 Mpc H-alpha and Ultraviolet Galaxy Survey(11HUGS), which has obtained H-alpha fluxes from ground-based narrowbandimaging, and UV fluxes from imaging with GALEX. For normal spiral galaxies(SFR~1 M_sun/yr), our results are consistent with previous work which has shownthat FUV SFRs tend to be lower than H-alpha SFRs before accounting for internaldust attenuation, but that there is relative consistency between the twotracers after proper corrections are applied. However, a puzzle is encounteredat the faint end of the luminosity function. As lower luminosity dwarfgalaxies, roughly less active than the Small Magellanic Cloud, are examined,H-alpha tends to increasingly under-predict the SFR relative to the FUV.Although past studies have suggested similar trends, this is the first timethis effect is probed with a statistical sample for galaxies with SFR~<0.1M_sun/yr. A range of standard explanations does not appear to be able toaccount for the magnitude of the systematic. Some recent work has argued for anIMF which is deficient in high mass stars in dwarf and low surface brightnessgalaxies, and we also consider this scenario.

Comparison of H-alpha and UV Star Formation Rates in the Local Volume: Systematic Discrepancies for Dwarf Galaxies