A Variant Stellar-to-nebular Dust Attenuation Ratio on Subgalactic and Galactic Scales

The state-of-the-art geometry models of stars/dust suggest that dust attenuation toward nebular regions ( ) is always larger than that of stellar regions ( ). Utilizing the newly released integral field spectroscopic data from the Mapping Nearby Galaxies at Apache Point Observatory survey, we investigate whether and how the ratio varies from subgalactic to galactic scales. On a subgalactic scale, we report a stronger correlation between and for more active H ii regions. The local is found to have moderate nonlinear correlations with three tracers of diffuse ionized gas (DIG), as well as indicators of gas-phase metallicity and ionization. The DIG regions tend to have larger compared to classic H ii regions excited by young OB stars. Metal-poor regions with a higher ionized level suffer much less nebular attenuation and thus have larger ratios. A low- and high- sequence, which can be resolved into DIG-dominated and metal-poor regions, on the three BPT diagrams is found. Based on these observations, we suggest that besides the geometry of stars/dust, local physical conditions such as metallicity and ionized level also play an important role in determining the . On a galactic scale, the global ratio has strong correlations with stellar mass (M*), moderate correlations with star formation rate (SFR) and metallicity, and weak correlations with inclination and specific SFR. Galaxies with larger M* and higher SFR that are more metal-rich tend to have smaller ratios. Such correlations form a decreasing trend of along the star-forming main sequence and mass-metallicity relation. The dust growth process accompanied by galaxy growth might be one plausible explanation for our observations.