RUBIES: JWST/NIRSpec Resolves Evolutionary Phases of Dusty Star-forming Galaxies at <i>z</i> ∼ 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 similar to 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 (log(10) L-IR/L-circle dot > 12.4), massive (log10 M-star/M-circle dot > 11), and very dust obscured (A(V) similar to 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 similar to 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.