Interpreting the Star Formation–Extinction Relation with MaNGA

Abstract

We investigate the resolved relation between local extinction and star formation surface density within nearby star-forming galaxies selected from the MaNGA survey. Balmer decrement measurements imply an extinction of the H α line emission that scales approximately linearly with the logarithm of the star formation surface density: . Secondary dependencies are observed such that, at a given Σ SFR , regions of lower metallicity and/or enhanced H α equivalent width (EW) suffer less obscuration than regions of higher metallicity and/or lower H α EW. Spaxels lying above the mean relation also tend to belong to galaxies that are more massive, larger, and viewed under higher inclination than average. We present a simple model in which the observed trends can be accounted for by a metallicity-dependent scaling between Σ SFR and Σ dust via a superlinear Kennicutt–Schmidt relation ( n KS  ∼ 1.47) and a dust-to-gas ratio that scales linearly with metallicity (DGR( Z ⊙ ) = 0.01). The relation between the resulting total dust column and observed effective extinction toward nebular regions requires a geometry for the relative distribution of H α -emitting regions and dust that deviates from a uniform foreground screen and also from an entirely homogeneous mixture of dust and emitting sources. The best-fit model features an H α EW and galactocentric-distance-dependent fraction of the dust mass in a clumpy foreground screen in front of a homogeneous mixture.