ALMA resolves the molecular gas in a young low-metallicity starburst
  galaxy at z=1.7
  We present Atacama Large Millimeter/submillimeter Array observations of CO
lines and dust continuum emission of the source RCSGA 032727--132609, a young
$z=1.7$ low-metallicity starburst galaxy. The CO(3-2) and CO(6-5) lines, and
continuum at rest-frame $450\,\mu m$ are detected and show a resolved structure
in the image plane. We use the corresponding lensing model to obtain a source
plane reconstruction of the detected emissions revealing intrinsic flux density
of $S_{450\,\mu m}=23.5_{-8.1}^{+26.8}$ $\mu$Jy and intrinsic CO luminosities
$L'_{\rm CO(3-2)}=2.90_{-0.23}^{+0.21}\times10^{8}$ ${\rm
K\,km\,s^{-1}\,pc^{2}}$ and $L'_{\rm CO(6-5)}=8.0_{-1.3}^{+1.4}\times10^{7}$
${\rm K\,km\,s^{-1}\,pc^{2}}$. We used the resolved properties in the source
plane to obtain molecular gas and star-formation rate surface densities of
$\Sigma_{\rm H2}=16.2_{-3.5}^{+5.8}\,{\rm M}_{\odot}\,{\rm pc}^{-2}$ and
$\Sigma_{\rm SFR}=0.54_{-0.27}^{+0.89}\,{\rm M}_{\odot}\,{\rm yr}^{-1}\,{\rm
kpc}^{-2}$ respectively. The intrinsic properties of RCSGA 032727--132609 show
an enhanced star-formation activity compared to local spiral galaxies with
similar molecular gas densities, supporting the ongoing merger-starburst phase
scenario. RCSGA 032727--132609 also appears to be a low--density starburst
galaxy similar to local blue compact dwarf galaxies, which have been suggested
as local analogs to high-redshift low-metallicity starburst systems. Finally,
the CO excitation level in the galaxy is consistent with having the peak at
${\rm J}\sim5$, with a higher excitation concentrated in the star-forming
clumps.
