Predicting dust extinction properties of star-forming galaxies from
  H-alpha/UV ratio
  Using star-forming galaxies sample in the nearby Universe (0.02<z<0.10)
selected from the SDSS (DR7) and GALEX all-sky survey (GR5), we present a new
empirical calibration for predicting dust extinction of galaxies from
H-alpha-to-FUV flux ratio. We find that the H-alpha dust extinction (A(Ha))
derived with H-alpha/H-beta ratio (Balmer decrement) increases with increasing
H-alpha/UV ratio as expected, but there remains a considerable scatter around
the relation, which is largely dependent on stellar mass and/or H-alpha
equivalent width (EW(Ha)). At fixed H-alpha/UV ratio, galaxies with higher
stellar mass (or galaxies with lower EW(Ha)) tend to be more highly obscured by
dust. We quantify this trend and establish an empirical calibration for
predicting A(Ha) with a combination of H-alpha/UV ratio, stellar mass and
EW(Ha), with which we can successfully reduce the systematic uncertainties
accompanying the simple H-alpha/UV approach by ~15-30%. The new recipes
proposed in this study will provide a convenient tool for predicting dust
extinction level of galaxies particularly when Balmer decrement is not
available. By comparing A(Ha) (derived with Balmer decrement) and A(UV)
(derived with IR/UV luminosity ratio) for a subsample of galaxies for which
AKARI FIR photometry is available, we demonstrate that more massive galaxies
tend to have higher extra extinction towards the nebular regions compared to
the stellar continuum light. Considering recent studies reporting smaller extra
extinction towards nebular regions for high-redshift galaxies, we argue that
the dust geometry within high-redshift galaxies resemble more like low-mass
galaxies in the nearby Universe.
