Milky Way–like Gas Excitation in an Ultrabright Submillimeter Galaxy at z = 1.6

Based on observations with the IRAM 30 m and Yebes 40 m telescopes, we report evidence of the detection of Milky Way–like, low-excitation molecular gas, up to the transition CO(J = 5–4), in a distant, dusty star-forming galaxy at zCO = 1.60454. WISE J122651.0+214958.8 (alias SDSS J1226, the Cosmic Seahorse), is strongly lensed by a foreground galaxy cluster at z = 0.44 with a source magnification of μ = 9.5 ± 0.7. This galaxy was selected by cross-correlating near-to-mid-infrared colors within the full-sky AllWISE survey, originally aiming to discover rare analogs of the archetypical strongly lensed submillimeter galaxy SMM J2135–0102, the Cosmic Eyelash. We derive an apparent (i.e., not corrected for lensing magnification) rest-frame 8–1000 μm infrared luminosity of $\mu {\text{}}{L}_{\mathrm{IR}}={1.66}_{-0.04}^{+0.04}\times {10}^{13}$ L⊙ and apparent star formation rate μSFRIR = 2960 ± 70 M⊙ yr−1. SDSS J1226 is ultrabright at S350μm ≃ 170 mJy and shows similarly bright low-J CO line intensities as SMM J2135–0102, however, with exceptionally small CO(J = 5–4) intensity. We consider different scenarios to reconcile our observations with typical findings of high-z starbursts, and speculate about the presence of a previously unseen star formation mechanism in cosmic noon submillimeter galaxies. In conclusion, the remarkable low line luminosity ratio r5,2 = 0.11 ± 0.02 is best explained by an extended, main-sequence star formation mode—representing a missing link between starbursts to low-luminosity systems during the epoch of peak star formation history.