2025ApJ...982..125C

The dearth of high-quality spectroscopy of dusty star-forming galaxies (DSFGs)-the main drivers of the assembly of dust and stellar mass at the peak of activity in the Universe-greatly hinders our ability to interpret their physical processes and evolutionary pathways. We present JWST/NIRSpec observations from RUBIES of four submillimeter-selected, Atacama Large Millimeter/submillimeter Array (ALMA)-detected DSFGs at cosmic noon, z ∼ 2.3-2.7. While photometry uniformly suggests vigorous ongoing star formation for the entire sample in line with canonical DSFGs, the spectra differ: one source has spectroscopic evidence of an evolved stellar population, indicating a recent transition to a post-starburst phase, while the remainder show strong spectroscopic signatures of ongoing starbursts. All four galaxies are infrared luminous (log₁₀L_IR/L_⊙ > 12.4), massive (log₁₀M_⋆/M_⊙ > 11), and very dust obscured (A_V ∼ 3-4 ABmag). Leveraging detections of multiple Balmer and Paschen lines, we derive an optical attenuation curve consistent with Calzetti overall, yet an optical extinction ratio R_V ∼ 2.5, potentially indicating smaller dust grains or differences in star-dust geometry. This case study provides some of the first detailed spectroscopic evidence that the DSFGs encompass a heterogeneous sample spanning a range of star formation properties and evolutionary stages, and illustrates the advantages of synergistic JWST and ALMA analysis of DSFGs.