ALMA resolves the molecular gas in a young low-metallicity starburst galaxy at z=1.7

Abstract: 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.