Comparison of H-alpha and UV Star Formation Rates in the Local Volume:
  Systematic Discrepancies for Dwarf Galaxies
  (abridged) Using a complete sample of ~300 star-forming galaxies within 11
Mpc, we evaluate the consistency between star formation rates (SFRs) inferred
from the far ultraviolet (FUV) non-ionizing continuum and H-alpha nebular
emission, assuming standard conversion recipes in which the SFR scales linearly
with luminosity at a given wavelength. Our analysis probes SFRs over 5 orders
of 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 narrowband
imaging, 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 shown
that FUV SFRs tend to be lower than H-alpha SFRs before accounting for internal
dust attenuation, but that there is relative consistency between the two
tracers after proper corrections are applied. However, a puzzle is encountered
at the faint end of the luminosity function. As lower luminosity dwarf
galaxies, 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 time
this effect is probed with a statistical sample for galaxies with SFR~<0.1
M_sun/yr. A range of standard explanations does not appear to be able to
account for the magnitude of the systematic. Some recent work has argued for an
IMF which is deficient in high mass stars in dwarf and low surface brightness
galaxies, and we also consider this scenario.
