Hydrogen recombination lines, one of the strongest emission lines from star-forming galaxies, are used to probe the early Universe as indicators of star formation rates and ionizing photon production rates. Ratios between different recombination lines provide clues for estimating dust attenuation, in addition to the HII region physical diagnostics. To prepare for future near-infrared spectral surveys and optical narrow-band imaging surveys aiming for hydrogen lines at different redshifts, we construct hydrogen recombination line libraries by compiling data from published literature. Our compilation includes 260 galaxies mostly at z ≲ 0.4, of which at least one hydrogen recombination line is observed. The specific hydrogen emission lines under investigation encompass three Balmer lines (Hα, Hβ and Hγ) and the Lyα line. Of the compiled galaxies, we estimated dust extinction for 85 galaxies that have detections of at least two Balmer lines, by assuming Case B recombination and Calzetti extinction curve, based on which Lyα escape fraction is also inferred. For each of the 14 galaxies with detections of all three Balmer lines, we optimized the extinction curve so that it yields consistent E(B − V ) values across the three possible Balmer line ratios, showing the variety in the slope of the dust attenuation curve. We further evaluated the feasibility of studying hydrogen emission line-selected galaxies with future spectral surveys such as SPHEREx by modeling the Paschen lines of our galaxy sample.

Introduction
Data
Dust Attenuation Properties from Balmer Lines
    Dust Attenuation and Lyα Escape Fraction
    Constraints on the Shape of the Dust Attenuation Curve
Detectability of Hydrogen Lines with SPHEREx
    SPHEREx Mock SEDs with Paschen Lines
    Measurement of Paschen Lines in Mock SEDs
Summary
Acknowledgments
References
Summary of the Compiled Galaxy Properties
Examples of SPHEREx Mock Paschen Lines

• Junho Song(Department of Earth Environmental and Space Convergence Sciences, Chungnam National University, Daejeon 34134, Republic of Korea)
• Hyunmi Song(Department of Astronomy and Space Science, Chungnam National University, Daejeon 34134, Republic of Korea) Corresponding author
• Hyunjin Shim(Department of Earth Science Education, Kyungpook National University, Daegu 41566, Republic of Korea)