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

TL;DR

This study compares IRX and Balmer decrement as dust attenuation indicators in 32,000 local star-forming galaxies, revealing their discrepancy depends on specific star formation rate and galaxy size, not metallicity or axial ratio.

Contribution

It introduces a scaling relation linking nebular and stellar attenuation discrepancies to galaxy size and SSFR, enhancing dust correction accuracy.

Findings

IRX is independent of stellar mass at fixed Hα/Hβ

Discrepancy depends on SSFR and galaxy size, not metallicity

A scaling relation allows better dust attenuation estimates

Abstract

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.