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Sub-kiloparsec imaging of cool molecular gas in two strongly lensed dusty, star-forming galaxies

  • Author(s): Spilker, JS;
  • Aravena, M;
  • Marrone, DP;
  • Béthermin, M;
  • Bothwell, MS;
  • Carlstrom, JE;
  • Chapman, SC;
  • Collier, JD;
  • Breuck, CD;
  • Fassnacht, CD;
  • Galvin, T;
  • Gonzalez, AH;
  • González-López, J;
  • Grieve, K;
  • Hezaveh, Y;
  • Ma, J;
  • Malkan, M;
  • O'Brien, A;
  • Rotermund, KM;
  • Strandet, M;
  • Vieira, JD;
  • Weiss, A;
  • Wong, GF
  • et al.
Abstract

We present spatially resolved imaging obtained with the Australia Telescope Compact Array (ATCA) of three CO lines in two high-redshift gravitationally lensed dusty star-forming galaxies, discovered by the South Pole Telescope. Strong lensing allows us to probe the structure and dynamics of the molecular gas in these two objects, at z = 2.78 and z = 5.66, with effective source-plane resolution of less than 1 kpc. We model the lensed emission from multiple CO transitions and the dust continuum in a consistent manner, finding that the cold molecular gas as traced by low-J CO always has a larger half-light radius than the 870 μm dust continuum emission. This size difference leads to up to 50% differences in the magnification factor for the cold gas compared to dust. In the z = 2.78 galaxy, these CO observations confirm that the background source is undergoing a major merger, while the velocity field of the other source is more complex. We use the ATCA CO observations and comparable resolution Atacama Large Millimeter/submillimeter Array dust continuum imaging of the same objects to constrain the CO-H2 conversion factor with three different procedures, finding good agreement between the methods and values consistent with those found for rapidly star-forming systems. We discuss these galaxies in the context of the star formation - gas mass surface density relation, noting that the change in emitting area with observed CO transition must be accounted for when comparing high-redshift galaxies to their lower redshift counterparts.

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