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

Abstract

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 z _CO = 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 $\mu {\text{}}{L}_{\mathrm{IR}}={1.66}_{-0.04}^{+0.04}\times {10}^{13}$ μLIR=1.66-0.04+0.04×1013 L _⊙ and apparent star formation rate μSFR_IR = 2960 ± 70 M _⊙ yr^-1. SDSS J1226 is ultrabright at S _350μm ≃ 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 r _5,2 = 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.