RUBIES: JWST/NIRSpec Resolves Evolutionary Phases of Dusty Star-forming Galaxies at z ∼ 2

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 (log₁₀ L_IR/L_⊙ > 12.4), massive (log₁₀ M_⋆/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.