Differences in the spatial distributions of stars and ionized gas relative to dust in galaxies imply that stellar and nebular dust attenuation curves need not be identical. Although stellar attenuation curves have been extensively studied, nebular attenuation curves remain less explored. In this work, we investigate nebular attenuation curves using Balmer line ratios for a sample of 90,958 nearby star-forming galaxies from the Sloan Digital Sky Survey with well-constrained stellar attenuation curves. In the Hα/Hβ versus Hα/Hγ diagram, the nebular attenuation curves are, on average, significantly shallower than typical stellar attenuation curves, a difference that can be qualitatively reproduced by source-dust geometric effects. The nebular attenuation curve slopes show weak positive correlations with the stellar attenuation curve slopes and weak dependences on galaxy properties that broadly follow those of the stellar attenuation curve slopes. These results suggest that nebular attenuation curves are distinct from stellar attenuation curves, while remaining consistent with a connection between them.

Introduction
Data and Sample
    Catalogs
    Sample Selection
Results and Discussion
    Constraining Nebular Attenuation Curve
    Effect of Dust-Covering Fraction
    Dependence on Physical Parameters
Summary
Acknowledgments
References
Effect of Screen-Mixed Geometry

• Jong Chul Lee(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea) Corresponding author
• Woong-Seob Jeong(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea)
• Jong-Ho Shinn(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea)
• Hyunjin Shim(Department of Earth Science Education, Kyungpook National University, Daegu 41566, Republic of Korea)
• Hyunmi Song(Department of Astronomy and Space Science, Chungnam National University, Daejeon 34134, Republic of Korea)
• Ho Seong Hwang(Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea, SNU Astronomy Research Center, Seoul National University, Seoul 08826, Republic of Korea, Institute for Data Innovation in Science, Seoul National University, Seoul 08826, Republic of Korea)