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 μm 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 an intrinsic flux density of {S}_{450μ {{m}}}={23.5}_-8.1^+26.8 μJy and intrinsic CO luminosities {L}_{CO(3-2)}^{\prime }={2.90}_-0.23^+0.21 × {10}⁸ {{K}} {km} {{{s}}}^-1 {{pc}}² and {L}_{CO(6-5)}^{\prime }={8.0}_-1.3^+1.4× {10}⁷ {{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 {{{Σ }}}_{{H}2}={16.2}_-3.5^+5.8 {M}_⊙ {{pc}}^-2 and {{{Σ }}}_{SFR}={0.54}_-0.27^+0.89 {M}_⊙ {{yr}}^-1 {{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 J∼ 5, with a higher excitation concentrated in the star-forming clumps.