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

Understanding the Discrepancy between IRX and Balmer Decrement in Tracing Galaxy Dust Attenuation