Understanding the Discrepancy between IRX and Balmer Decrement in
  Tracing Galaxy Dust Attenuation
  We compare the infrared excess (IRX) and Balmer decrement (${\rm
H\alpha/H\beta }$) as dust attenuation indicators in relation to other galaxy
parameters using a sample of $\sim$32 000 local star-forming galaxies (SFGs)
carefully selected from SDSS, GALEX and WISE. While at fixed ${\rm
H\alpha/H\beta }$, IRX turns out to be independent on galaxy stellar mass, the
Balmer decrement does show a strong mass dependence at fixed IRX. We find the
discrepancy, parameterized by the color excess ratio $R_{\rm EBV} \equiv
E(B-V)_{\rm IRX}/E(B-V)_{\rm H\alpha/H\beta }$, is not dependent on the
gas-phase metallicity and axial ratio but on the specific star formation rate
(SSFR) and galaxy size ($R_{\rm e}$) following $R_{\rm EBV}=0.79+0.15\log({\rm
SSFR}/R_{\rm e}^{2})$. This finding reveals that the nebular attenuation as
probed by the Balmer decrement becomes increasingly larger than the global
(stellar) attenuation of SFGs with decreasing SSFR surface density. This can be
understood in the context of an enhanced fraction of intermediate-age stellar
populations that are less attenuated by dust than the HII region-traced young
population, in conjunction with a decreasing dust opacity of the diffuse ISM
when spreading over a larger spatial extent. Once the SSFR surface density of
an SFG is known, the conversion between attenuation of nebular and stellar
emission can be well estimated using our scaling relation.
