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 $L_{\mathrm{C}\mathrm{O}(3-2)}^{\mathrm{\prime }}=2.90_{-0.23}^{+0.21}\times {10}^8$ ${\rm K,km,s^{-1},pc^{2}}$and$L’{\rm CO(6-5)}=8.0{-1.3}^{+1.4}\times10^{7}$ $\mathrm{K}\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2$. We used the resolved properties in the source plane to obtain molecular gas and star-formation rate surface densities of ${\mathrm{\Sigma }}_{\mathrm{H}2}=16.2_{-3.5}^{+5.8}{\mathrm{M}}_{\odot }{\mathrm{p}\mathrm{c}}^{-2}$ 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 $\mathrm{J}\sim 5$, with a higher excitation concentrated in the star-forming clumps.