Evidence for Shallow Nebular Attenuation Curves and Patchy Dust Geometry at z~2 with Pa-beta/H-alpha Measurements from JWST-MegaScience Medium Band Photometry

TL;DR

This work addresses how dust geometry and the nebular attenuation curve shape the emission-line properties of star-forming galaxies at $z\sim2$. By measuring $\mathrm{Pa}\beta/\mathrm{H}\alpha$ from JWST medium-band photometry in 66 galaxies (and $\mathrm{H}\alpha$ for 143 more), and comparing to Balmer decrements from a mass-, SFR-, and redshift-matched MOSDEF sample, the authors constrain the attenuation curve, finding support for the shallower curve from $\mathrm{reddy\ et\ al.\ 2025}$ over Cardelli et al. (1989) for high-mass systems. They report $0\le A_{\mathrm{H}\alpha,\mathrm{neb}}\le 4.58$ mag with a median of $1.24$ mag under the reddy curve, and reveal no trend of nebular attenuation with axis ratio, implying a patchy, clumpy dust geometry with attenuation localized to small regions. These results imply substantial revisions to SFR inferences and demonstrate the effectiveness of JWST medium-band photometry for large-scale dust studies at cosmic noon, motivating even larger surveys like MINERVA. The findings emphasize that dust geometry, specifically the covering fraction and clumpiness, can drive shallower attenuation curves and impact derived galaxy properties by factors up to ~2 for $H\alpha$-based SFRs. Key contributions include (i) direct photometric measurements of $\mathrm{H}\alpha$ and $\mathrm{Pa}\beta$ across a wide mass range at $1.2<z<2.4$, (ii) robust, interval-censored inferences of the nebular attenuation curve via cross-sample comparison, and (iii) evidence for patchy dust geometry dominating nebular attenuation in massive galaxies at cosmic noon.

Abstract

We constrain the nebular attenuation curve and investigate dust geometry in star-forming galaxies at cosmic noon using photometric medium-band emission line measurements. We measure H-alpha emission line fluxes for a sample of 209 star-forming galaxies at 1.2<z<2.4 in MegaScience/UNCOVER with stellar masses spanning $7.85<\log_{10}(M_*/M_\odot)<11.0$. For 66 of these galaxies, we also measure a Pa-beta flux. We find that the Pa-beta/H-alpha line ratio increases strongly with stellar mass and star-formation rate (SFR) across our full mass range, indicating that more massive galaxies are dustier. We compare our results with a mass-, SFR-, and redshift-matched sample of galaxies from the MOSDEF survey with spectroscopic measurements of H-alpha/H-beta, finding that a shallow Reddy et al. (2025) nebular attenuation curve is more consistent with our observations than the typically assumed Cardelli et al. (1989) attenuation curve, especially for massive galaxies. This shallow attenuation curve could be explained by low dust covering fractions in star-forming regions. Through comparison to other studies, we show that assuming this shallower attenuation curve can increase the inferred A_Halpha,neb by up to 1 magnitude at high masses. We observe no trend between A_Halpha,neb and axis ratio, indicating that nebular attenuation is likely localized to small clumps. Altogether, our results strongly suggest that dust geometry is patchy and non-uniform, especially in massive galaxies. Our results highlight the ability of JWST medium bands to probe emission lines for large samples of galaxies, and statistically constrain dust properties in upcoming large programs.

Figures (5)

• Figure 1: Prospector SFR vs stellar mass for the 66 galaxies in our sample with both $\textrm{H}\alpha$ and $\textrm{Pa}\beta$ measurements (circles, colored by redshift, green histogram), the 143 galaxies with $\textrm{H}\alpha$ detections and $\textrm{Pa}\beta$ upper limits (squares, black histogram), and the full UNCOVER sample (gray hexagons, gray histogram). Histograms for SFR and stellar mass for the three populations are also shown. Our sample captures galaxies down to $\log_{10}(M_*/M_\odot) = 7.85$. They span the full range of redshifts from $1.2<z<2.4$, and do not exhibit mass or SFR trends with redshift.
• Figure 2: Measured $\textrm{Pa}\beta$/$\textrm{H}\alpha$ line ratio plotted against stellar mass (left) and SFR (right). A larger line ratio implies a more dusty galaxy, although the exact conversion depends on the assumed attenuation curve. The intrinsic $\textrm{Pa}\beta$/$\textrm{H}\alpha$ ratio assuming Case B recombination is 1/18 (0.056). Medians and uncertainties (orange) are computed in bins with equal numbers of galaxies from the fully-detected sample using the turnbull_empirical_1976 estimator that accounts for the upper limits in each bin. As expected, we see a clear trend of increasing dust attenuation with increasing mass and increasing SFR.
• Figure 3: MegaScience $\textrm{Pa}\beta$/$\textrm{H}\alpha$ line ratio vs. the $\textrm{H}\alpha$/$\textrm{H}\beta$ for a comparable sample of MOSDEF galaxies (see Section \ref{['subsec:mosdef_sample']}). We present median line ratios from four mass bins that contain roughly equal numbers of MOSDEF galaxies, and their uncertainties are computed from bootstrapping. Predicted line ratios from the reddy_jwstaurora_2025 and cardelli_relationship_1989 nebular attenuation curves are shown. The data are consistent with the reddy_jwstaurora_2025 attenuation curve, particularly at high stellar masses. This consistency suggests that a shallower slope than cardelli_relationship_1989 is required at this mass and redshift regime.
• Figure 4: Median inferred $A_{\mathrm{\textrm{H}\alpha\xspace, \mathrm{neb}}}$ values for MegaScience (orange) compared to similar studies of star-forming galaxies at cosmic noon (CANUCS, MOSDEF, and Blue Jay). All other works use $\textrm{H}\alpha$/$\textrm{H}\beta$ line ratios, while MegaScience uses the redder $\textrm{Pa}\beta$/$\textrm{H}\alpha$ line ratio. The $A_{\mathrm{\textrm{H}\alpha\xspace, \mathrm{neb}}}$ are calculated assuming the cardelli_relationship_1989 attenuation curve (left) and the reddy_jwstaurora_2025 attenuation curve (right). The green shaded region shows the 1$\sigma$ fit to the comparison samples. We see that the cardelli_relationship_1989 attenuation curve causes our MegaScience measurements to be biased systematically high (median offset: 0.50mag), while the reddy_jwstaurora_2025 has a smaller median offset (0.34mag), especially at higher masses. We note that the different attenuation curves can cause wide variations in inferred $A_{\mathrm{\textrm{H}\alpha\xspace, \mathrm{neb}}}$ for the Balmer decrement samples, with over 1 magnitude of difference in the highest mass Blue Jay bin.
• Figure 5: Measured $\textrm{Pa}\beta$/$\textrm{H}\alpha$ line ratio vs axis ratio, as determined by Pysersic fits to the photometry. We do not find a trend between the line ratio and axis ratio as measured in F150W (shown), F444W, or the medium band containing $\textrm{H}\alpha$. Medians including the upper limits are shown in orange, with uncertainties measured through bootstrapping. The absence of any trend between dust content and inclination indicates that edge-on galaxies show similar nebular dust attenuation to face-on galaxies.