COMPARISON OF Hα AND UV STAR FORMATION RATES IN THE LOCAL VOLUME: SYSTEMATIC DISCREPANCIES FOR DWARF GALAXIES

Abstract

(abridged) Using a complete sample of ~300 star-forming galaxies within 11\nMpc, we evaluate the consistency between star formation rates (SFRs) inferred\nfrom the far ultraviolet (FUV) non-ionizing continuum and H-alpha nebular\nemission, assuming standard conversion recipes in which the SFR scales linearly\nwith luminosity at a given wavelength. Our analysis probes SFRs over 5 orders\nof magnitude, down to ultra-low activities on the order of ~0.0001 M_sun/yr.\nThe data are drawn from the 11 Mpc H-alpha and Ultraviolet Galaxy Survey\n(11HUGS), which has obtained H-alpha fluxes from ground-based narrowband\nimaging, and UV fluxes from imaging with GALEX. For normal spiral galaxies\n(SFR~1 M_sun/yr), our results are consistent with previous work which has shown\nthat FUV SFRs tend to be lower than H-alpha SFRs before accounting for internal\ndust attenuation, but that there is relative consistency between the two\ntracers after proper corrections are applied. However, a puzzle is encountered\nat the faint end of the luminosity function. As lower luminosity dwarf\ngalaxies, roughly less active than the Small Magellanic Cloud, are examined,\nH-alpha tends to increasingly under-predict the SFR relative to the FUV.\nAlthough past studies have suggested similar trends, this is the first time\nthis effect is probed with a statistical sample for galaxies with SFR~<0.1\nM_sun/yr. A range of standard explanations does not appear to be able to\naccount for the magnitude of the systematic. Some recent work has argued for an\nIMF which is deficient in high mass stars in dwarf and low surface brightness\ngalaxies, and we also consider this scenario.\n