Balmer Decrements and Nebular-Stellar Reddening in JADES Galaxies at $2.7<z<7$

Shreya Karthikeyan; Leonardo Clarke; Alice E. Shapley; Natalie Lam; Ryan L. Sanders; Naveen A. Reddy; Michael W. Topping; Gabriel B. Brammer

arXiv · 2603.11338

Balmer Decrements and Nebular-Stellar Reddening in JADES Galaxies at $2.7<z<7$

Shreya Karthikeyan·Leonardo Clarke·Alice E. Shapley·Natalie Lam·Ryan L. Sanders·Naveen A. Reddy·Michael W. Topping·Gabriel B. Brammer

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Abstract

We aim to characterize nebular and stellar reddening in star-forming galaxies as a function of global galaxy properties (stellar mass, SFR, metallicity) at $2.7 < z< 7.0$. We also provide a prescription to convert SED-based $E(B-V)_{\mathrm{star}}$ to $E(B-V)_{\mathrm{gas}}$ when direct measurements of nebular reddening are unavailable. Our results are based on JWST/NIRSpec measurements of both individual spectra, with a sample of 283 galaxies, and composite spectra, including a larger sample of 327 galaxies. We estimate nebular reddening using the Balmer decrement (H$\alpha$/H$\beta$) above $10^{8.5}$ $M_{\odot}$, where the sample is representative. Stellar reddening and SFRs are derived through Prospector SED fitting, while gas-phase metallicities are estimated from strong emission-line ratios. At fixed stellar mass, Balmer decrements remain consistent within uncertainties across our redshift range, indicating that stellar mass primarily determines the overall dust column even by $z \sim 7$. We find that differential reddening ($\Delta E(B-V) \equiv E(B-V)_{\mathrm{gas}} - E(B-V)_{\mathrm{star}}$) scales linearly with mass and SFR at $z \sim 2.7 - 4.0$, but shows no mass or SFR dependence above $z \sim 4.0$. We find evidence for smaller $\Delta E(B-V)$ above $z \sim 5.0$, suggesting that nebular emission and stellar continuum probe increasingly similar dust columns towards higher redshift. Finally, we find that nebular reddening correlates strongly with metallicity out to $z \sim 5$, whereas the correlation between stellar reddening and metallicity is weaker or absent. Together, these results suggest that both dust mass and geometry play a significant role in shaping the observed reddening of high-redshift galaxies.

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Shreya Karthikeyan, Leonardo Clarke, Alice E. Shapley, Natalie Lam, Ryan L. Sanders, Naveen A. Reddy, Michael W. Topping, Gabriel B. Brammer. 2026-03-11. Balmer Decrements and Nebular-Stellar Reddening in JADES Galaxies at $2.7<z<7$. https://arxiv.org/abs/2603.11338

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