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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 ($A_{V,gas}$) is always larger than that of stellar regions ($A_{V,star}$). Utilizing the newly released integral field spectroscopic data from the MaNGA survey, we investigate whether and how the $A_{V,star}/A_{V,gas}$ ratio varies from subgalactic to galactic scales. On a subgalactic scale, we report a stronger correlation between $A_{V,star}$ and $A_{V,gas}$ for more active HII regions. The local $A_{V,star}/A_{V,gas}$ 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 $A_{V,star}/A_{V,gas}$ compared to classic HII regions excited by young OB stars. Metal-poor regions with a higher ionized level suffer much less nebular attenuation and thus have larger $A_{V,star}/A_{V,gas}$ ratios. A low-$A_{V,gas}$ and high-$A_{V,star}/A_{V,gas}$ 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 $A_{V,star}/A_{V,gas}$. On a galactic scale, the global $A_{V,star}/A_{V,gas}$ ratio has strong correlations with stellar mass ($M_*$), moderate correlations with 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 $A_{V,star}/A_{V,gas}$ ratios. Such correlations form a decreasing trend of $A_{V,star}/A_{V,gas}$ 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.