2025ApJ...989..209S

We use JWST/NIRSpec observations from the Assembly of Ultradeep Rest-optical Observations Revealing Astrophysics survey to constrain the shape of the nebular attenuation curve of a star-forming galaxy at z = 4.41, GOODSN-17940. We utilize 12 H I recombination lines to derive the attenuation curve spanning optical to near-infrared wavelengths (3751-9550 Å). We then leverage a high signal-to-noise ratio spectroscopic detection of the rest-frame ultraviolet continuum in combination with rest-UV photometric measurements to constrain the shape of the curve at ultraviolet wavelengths. While this UV constraint is predominantly based on stellar emission, the large measured equivalent widths of Hα and Hβ indicate that GOODSN-17940 is dominated by an extremely young stellar population <10 Myr in age such that the UV stellar continuum experiences similar attenuation to that of the nebular emission. The resulting combined nebular attenuation curve spans 1400-9550 Å and has a shape that deviates significantly from commonly assumed dust curves in high-redshift studies. Relative to the Milky Way, SMC, and Calzetti curves, the new curve has a steeper slope at long wavelengths (λ > 5000 Å) while displaying a similar slope across blue-optical wavelengths (λ = 3750-5000 Å). In the ultraviolet, the new curve is shallower than the SMC and Calzetti curves and displays no significant 2175 Å bump. This work demonstrates that the most commonly assumed dust curves are not appropriate for all high-redshift galaxies. These results highlight the ability to derive nebular attenuation curves for individual high-redshift sources with deep JWST/NIRSpec spectroscopy, thereby improving the accuracy of physical properties inferred from nebular emission lines.