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
$z_{CO}=1.60454$. 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 $\mu=9.5\pm0.7$. 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
$\mu$m infrared luminosity of $\mu L_\mathrm{IR}=1.66^{+0.04}_{-0.04}\times
10^{13}$ L$_\odot$ and apparent star-formation rate
$\mu\mathrm{SFR}_\mathrm{IR}=2960\pm70$ M$_\odot$ yr$^{-1}$. SDSSJ1226 is
ultra-bright at $S_{350\mu m}\simeq170$ 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
$r_{5,2}=0.11\pm0.02$ 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.
