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 SDSSJ1226, the Cosmic Seahorse), is strongly lensed by a foreground galaxy cluster at $z=0.44$ with a source magnification of $\hat{I}¼=9.5\pm 0.7$. This galaxy was selected by cross-correlating near-to-mid infrared colours 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 $\hat{I}¼$m infrared luminosity of $\hat{I}¼L_{\mathrm{IR}}=1.66_{-0.04}^{+0.04}\times 10^{13}$ L${}_{\odot }$ and apparent star-formation rate $\hat{I}¼{\mathrm{SFR}}_{\mathrm{IR}}=2960\pm 70$ M${}_{\odot }$ yr${}^{-1}$. SDSSJ1226 is ultra-bright at $S_{350\hat{I}¼m}\simeq 170$ 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 $r_{5,2}=0.11\pm 0.02$ 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.

12 pages incl. one appendix, 5 figures, 2 tables. Accepted for publication in The Astrophysical Journal Letters