The AURORA Survey: Multiple Balmer and Paschen Emission Lines for Individual Star-forming Galaxies at <i>z</i>  = 1.5–4.4. I. A Diversity of Nebular Attenuation Curves and Evidence for Non-unity Dust Covering Fractions

Abstract We present the nebular attenuation curves and dust covering fractions for 24 z = 1.5–4.4 star-forming galaxies using multiple Balmer and Paschen lines from the JWST/AURORA survey. Nebular reddening derived from Paschen lines exceeds that from Balmer lines for at least half the galaxies in the sample when assuming the commonly adopted Galactic extinction curve, implying the presence of heavily reddened star formation. The nebular attenuation curves exhibit a broad range of normalizations ( R V ≃ 3.2–16.4). Motivated by the offsets in reddening deduced from the Balmer and Paschen lines, as well as the high R V values for the individual nebular attenuation curves, both of which suggest variations in the dust–star geometry, we propose a model with a sub-unity dust covering fraction ( f cov ). Fitting such a model to the H i recombination line ratios indicates f cov ∼ 0.6–1.0. The normalizations of the nebular attenuation curves, R V , are driven primarily by f cov and the mix of reddened and unreddened OB associations. Thus, the diversity of nebular attenuation curves can be accommodated by assuming dust grain properties similar to that of Milky Way sightlines but with a sub-unity covering fraction of dust. Integrated measurements of multiple Balmer and Paschen lines can be used to place novel constraints on the dust covering fraction towards OB associations. These, in turn, provide new avenues for exploring the role of the dust and gas covering fractions in a number of relevant aspects of high-redshift galaxies, including the impact of stellar feedback on ISM porosity and the escape of Ly α and Lyman continuum radiation.