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 $L_{\mathrm{CO}(3-2)}^{\prime }=2.90_{-0.23}^{+0.21}\times 10^8$ $\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2$ and $L_{\mathrm{CO}(6-5)}^{\prime }=8.0_{-1.3}^{+1.4}\times 10^7$ $\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2$. We used the resolved properties in the source plane to obtain molecular gas and star-formation rate surface densities of ${\Sigma }_{H2}=16.2_{-3.5}^{+5.8}{\mathrm{M}}_{\odot }{\mathrm{pc}}^{-2}$ and ${\Sigma }_{\mathrm{SFR}}=0.54_{-0.27}^{+0.89}{\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1}{\mathrm{kpc}}^{-2}$ 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.

11 pages, 4 figures, accepted for publication in ApJL

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• Predicting the resolved CO emission of $z=1-3$ star-forming galaxies