FRESCO: Constraining Dust Attenuation and Star-Formation Rates of $z\sim 2$ Star-Forming Galaxies with JWST Paschen and Ground-Based Balmer Emission Line Observations

Abstract

We present new constraints on dust attenuation and star-formation rates (SFRs) for 77 galaxies at redshifts $z=1.43-2.65$, using Paschen emission line detections from the JWST FRESCO survey and ground-based Balmer line measurements from the MOSDEF survey. Using nebular and continuum emission maps, we find that Paschen emission covers a smaller area than continuum emission observed in the F210M (2.1 $μ$m; rest-frame optical) and F444W (4.4 $μ$m; rest-frame near-IR) bands, and is preferentially located toward galaxy outskirts. These results suggest that current star formation is concentrated in regions farther from galaxy centers than older stellar populations traced by the continuum, indicative of inside-out star formation. With a careful accounting of slit-loss corrections for ground-based measurements, we calculate nebular reddening and dust-corrected SFRs using the Balmer decrement (H$α$/H$β$) and Paschen-to-Balmer line ratios (Pa$α$/H$α$ and Pa$β$/H$α$), assuming the Milky Way extinction curve. On average, Paschen-derived reddening and SFRs agree with Balmer-derived values; however, two galaxies exhibit significantly higher Paschen reddening and four show significantly higher Paschen SFRs. We find that non-unity dust covering fractions bias the Balmer decrement toward less reddened OB associations, while decrements involving the Paschen lines are less affected by this bias. These results highlight the enhanced sensitivity of the Paschen lines to the most heavily obscured OB associations in $z\sim2$ galaxies, particularly in galaxies with patchy dust geometries. Future studies using Paschen lines exclusively to measure nebular reddening will yield more robust constraints on the dustiest star-forming regions.

Figures (16)

• Figure 1: Spectroscopic redshift histogram of the galaxy sample used in the analysis. The lighter blue and red bars indicate all galaxies with Pa$\alpha$ (9 objects) and Pa$\beta$ (68 objects) coverage, respectively. The darker blue and red bars represent galaxies with ${\geq}3\sigma$ detections of Pa$\alpha$ (7 objects) and Pa$\beta$ (39 objects), respectively.
• Figure 2: Half-light areas derived from the Paschen and F444W+F210M continuum maps. The solid line indicates equality between the two measurements. Diamonds denote composite values obtained after sorting the full sample of 77 galaxies into three equal-sized bins based on Paschen half-light area. Paschen emission exhibits smaller areas than the continuum emission in all but one galaxy, with the composites likewise showing smaller Paschen emission areas.
• Figure 3: Differences between Paschen $M_{20}$ and continuum $M_{20}$ values plotted against the ratios of Paschen to continuum half-light areas. The dashed line represents the best-fit relation, with the shaded region showing the 68% confidence interval. Emission maps are shown for a selection of galaxies in the sample. For each galaxy, the left panel shows the continuum emission constructed from a stack of the F444W and F210M images, while the right panel shows the Paschen emission line map. Galaxies with more concentrated Paschen emission areas relative to the continuum tend to have Paschen emission located farther from the galaxy's center compared to the continuum.
• Figure 4: Ratios of revised slit-loss correction factors to those from the MOSDEF survey, plotted against the ratios of Paschen to continuum half-light areas. The dashed line represents the best-fit relation, with the shaded region showing the 68% confidence interval. Emission maps are shown for a selection of galaxies in the sample. For each galaxy, the left panel shows the continuum emission constructed from a stack of the F444W and F210M images, while the right panel shows the Paschen emission line map. Red lines mark the position of the slit during the MOSDEF observations, and 1$^{\prime\prime}$ scaling is shown in each continuum map. Galaxies with more concentrated Paschen emission areas relative to the continuum tend to have MOSDEF slit-loss corrections that overestimated the amount of excluded Balmer emission; for these galaxies, the slit contains most of the Paschen emission but excludes a significant fraction of the continuum.
• Figure 5: Balmer decrement compared to Pa$\alpha$/H$\alpha$ (left) and Pa$\beta$/H$\alpha$ (right), color-coded by SED-derived stellar mass. Large diamonds indicate values measured from composite spectra of all galaxies with the required line coverage. The red and blue lines show predictions when assuming the SMC and MW extinction curves, respectively, while dashed lines indicate line ratios in the absence of dust attenuation.