Star Formation and Nebular Attenuation from Pa$α$, Br$α$, and Br$β$ in Massive Dust-obscured Galaxies at Cosmic Noon using JWST

Virginia Vanicek, Jed McKinney, Alexandra Pope, Anna Sajina, Stacey Alberts, Meredith Stone, Lee Armus, Sinclaire M. Manning, Olivia Cooper, Tanio DÍaz-Santos, Miriam Eleazer, Caitlin Casey, Thomas S. -Y. Lai, Steven Finkelstein, Leonid Sajkov, Allison Kirkpatrick, Anthony Taylor

First listed 2026-09-22 | Last updated 2026-09-18

Abstract

Near-infrared hydrogen recombination lines can provide robust star-formation rates if the effects of dust attenuation continue to diminish towards longer wavelengths. To quantify near-infrared attenuation and ultimately measure accurate star-formation rates (SFRs) at the height of cosmic star-formation, we present Pa$α$, Br$α$, and Br$β$ detections in $27$ luminous infrared galaxies at $z\sim 1-2$, including pure star-forming galaxies as well as dust-obscure Active Galactic Nuclei (AGN). Using combined spectra from the JWST MIRI (Mid-Infrared Instrument) LRS (Low Resolution Spectrometer) and the Spitzer Space Telescope IRS we quantify the star formation rates of our sample using a sub-sample of 13 galaxies detected in Pa$α$ emission ($1.87\, μ$m), 14 galaxies detected in Br$α$ ($4.05\, μ$m), and 12 galaxies detected in Br$β$ ($2.63\, μ$m). By combining multiple near-infrared recombination lines in individual galaxies we calculate nebular attenuation, finding $A_λ\sim3$ mag at $\sim2\,μ$m and $A_λ\sim1$ mag at $\sim4\,μ$m. Nebular lines trace the SFR on $\sim 10$ Myr timescales, which we compare to the total infrared luminosity ($L_{\rm IR}$) which probes the star formation over the last $\sim 100$ Myr. SFRs measured from $L_{\rm IR}$ using common conversions are systematically higher than the SFRs derived from attenuation-corrected nebular line measurements by a factor of $\sim2.5$. This could be indicative of a declining or static star formation history with a past burst or systematic effects on $\rm SFR_{\rm IR}$ such as dust heating from evolved stellar populations, and/or an active galactic nucleus.

Short digest

Vanicek et al. use combined JWST/MIRI LRS and archival Spitzer/IRS spectra to detect Paα, Brα, and Brβ in 27 massive infrared-luminous galaxies at z≈1–2, spanning star-forming systems, obscured AGN, and composites. Multiple recombination-line measurements yield substantial nebular attenuation, with Aλ≈3 mag near 2 μm but still ≈1 mag near 4 μm, demonstrating that even near-IR SFR indicators require meaningful dust corrections in these galaxies. The attenuation-corrected, ≈10 Myr nebular SFRs agree across Paα, Brα, and Brβ, yet remain systematically lower than standard LIR-based, ≈100 Myr SFRs by a factor of about 2.5. The mismatch suggests that IR luminosities may retain signatures of earlier star formation or receive additional heating from evolved stars and/or an AGN, complicating their interpretation as instantaneous SFRs at cosmic noon.

Key figures to inspect

• Figure 5. This is the key attenuation result: it shows the line-by-line nebular extinction from Paα through Brα and directly establishes the strong wavelength dependence, including the contrast with continuum attenuation.
• Figure 7. This comparison demonstrates that Paα-, Brα-, and Brβ-based SFRs converge after the authors' attenuation corrections, providing the internal validation for their recombination-line methodology.
• Figure 8. This is the central timescale comparison, placing the short-timescale recombination-line SFRs against LIR-derived SFRs and visualizing the systematic offset that motivates the paper's physical interpretation.
• Figure 10. This synthesis figure shows the consequence for galaxy classification: nebular-line SFRs place the sources closer to the mass–redshift main sequence, whereas LIR SFRs would label them as exclusively starburst systems.