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

TL;DR

This study uses JWST Paschen and ground-based Balmer emission lines to better understand dust attenuation and star formation in galaxies at redshift around 2, revealing inside-out star formation and the importance of Paschen lines for obscured regions.

Contribution

It introduces a method combining Paschen and Balmer lines to improve dust and SFR measurements, highlighting the advantages of Paschen lines in dusty, patchy environments.

Findings

Paschen emission is more extended than continuum emission, indicating outside-in star formation.

Paschen-derived reddening and SFRs generally agree with Balmer-based estimates.

Paschen lines are more sensitive to heavily obscured star-forming regions.

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 $\mu$m; rest-frame optical) and F444W (4.4 $\mu$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$\alpha$/H$\beta$) and Paschen-to-Balmer line ratios (Pa$\alpha$/H$\alpha$ and Pa$\beta$/H$\alpha$), 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.