How Steep is your Dust? Direct Measurements of Nebular Attenuation Curves at Cosmic Noon with JWST

Abstract

Tracing the shape of the nebular attenuation curve, which is crucial for accurately measuring intrinsic emission-line fluxes, has proven challenging both in the local universe and at high redshift. Here we present nebular dust attenuation curves measured directly for 24 star-forming galaxies ($9.3<\log(\rm M{\star}/M_\odot)<11.1$) at cosmic noon ($1.8<z<3.3$) from the Blue Jay survey. Using deep JWST/NIRSpec spectra, we measure up to eight hydrogen recombination lines from H$δ$ to Pa$β$ per galaxy and find little diversity in the curve shape, with most galaxies agreeing with the \citet{calzetti_dust_2000} attenuation curve. For over $90\%$ of the sample, the nebular reddening from the Balmer lines is consistent with that from the Paschen lines. Using \texttt{Prospector} spectral energy distribution (SED) fits, we obtain the stellar reddening and find that the difference between stellar and nebular reddening increases with stellar mass, and that each reddening measurement is independent of inclination, which disagrees with the two-component dust model. We then measure the impact of using the derived nebular attenuation curve or a literature curve when determining metallicity using the strong-line method, and find that for some diagnostics (e.g., S32, S3, S23, and S3O3), the difference can be as large as $5σ$. Caution is needed when using these diagnostics if the nebular attenuation curve cannot be directly measured, since the resulting uncertainty can exceed the measurement uncertainty on the emission-line fluxes even with high signal-to-noise Balmer decrement measurements. These results show that the shape of the nebular attenuation curve is poorly understood and that constraining it is important for reducing systematic uncertainty in galaxy measurements.