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 (RV ≃ 3.2–16.4). Motivated by the offsets in reddening deduced from the Balmer and Paschen lines, as well as the high RV 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 (fcov). Fitting such a model to the H i recombination line ratios indicates fcov ∼ 0.6–1.0. The normalizations of the nebular attenuation curves, RV, are driven primarily by fcov 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 Lyα and Lyman continuum radiation.
The AURORA Survey: Multiple Balmer and Paschen Emission Lines for Individual Star-forming Galaxies at z = 1.5–4.4. I. A Diversity of Nebular Attenuation Curves and Evidence for Non-unity Dust Covering Fractions
Creators
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Reddy, Naveen A.1
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Shapley, Alice E.2
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Sanders, Ryan L.3
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Topping, Michael W.4
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Ellis, Richard S.5
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Pettini, Max6
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Brammer, Gabriel7
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Cullen, Fergus8
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Förster Schreiber, Natascha M.9
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Khostovan, Ali A.3
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McLeod, Derek J.8
- McLure, Ross J.8
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Narayanan, Desika10, 11
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Oesch, Pascal A.7, 12
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Pahl, Anthony J.13
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Steidel, Charles C.14
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Berg, Danielle A.15
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1.
University of California, Riverside
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2.
University of California, Los Angeles
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3.
University of Kentucky
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4.
University of Arizona
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University College London
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University of Cambridge
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University of Copenhagen
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Royal Observatory
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Max Planck Institute for Extraterrestrial Physics
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University of Florida
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Technical University of Denmark
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12.
University of Geneva
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Carnegie Observatories
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14.
California Institute of Technology
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15.
The University of Texas at Austin
Abstract
Copyright and License
© 2026. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
This work is based on observations made with the NASA/ ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute (MAST), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-03127 for JWST. The specific observations analyzed can be accessed from MAST via doi:10.17909/evwk-m381. We also acknowledge support from NASA grant No. JWST-GO-01914. Some of the data products used in this analysis were retrieved from the Dawn JWST Archive (DJA). The DJA is an initiative of the Cosmic Dawn Center, which is funded by the Danish National Research Foundation under grant DNRF140.
Facilities
JWST - James Webb Space Telescope.
Files
Reddy_2026_ApJ_999_15.pdf
Additional details
Related works
- Is new version of
- Discussion Paper: arXiv:2506.17396 (arXiv)
Funding
- National Aeronautics and Space Administration
- NAS5-03127
- Space Telescope Science Institute
- JWST-GO-01914
- Danish National Research Foundation
- DNRF140
Dates
- Submitted
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2025-06-18
- Accepted
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2026-01-10
- Available
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2026-02-20Published
Caltech Custom Metadata
- Caltech groups
- Astronomy Department , Physics Department , Division of Physics, Mathematics and Astronomy (PMA)
- Publication Status
- Published