An HI-absorption-selected cold rotating disk galaxy at $z\approx2.193$

We have used the Atacama Large Millimeter/submillimeter Array (ALMA) to map CO(3-2) emission from a galaxy, DLA-B1228g, associated with the high-metallicity damped Lyman-$\alpha$ absorber at $z \approx 2.1929$ towards the QSO PKS B1228-113. At an angular resolution of $\approx0.32''\times0.24''$, DLA-B1228g shows extended CO(3-2) emission with a deconvolved size of $\approx0.78''\times0.18''$, i.e. a spatial extent of $\approx6.4$ kpc. We detect extended stellar emission from DLA-B1228g in a Hubble Space Telescope Wide Field Camera 3 F160W image, and find that H$\alpha$ emission is detected in a Very Large Telescope SINFONI image from only one side of the galaxy. While the clumpy nature of the F160W emission and the offset between the kinematic and physical centers of the CO(3-2) emission are consistent with a merger scenario, this appears unlikely due to the lack of strong H$\alpha$ emission, the symmetric double-peaked CO(3-2) line profile, the high molecular gas depletion timescale, and the similar velocity dispersions in the two halves of the CO(3-2) image. Kinematic modelling reveals that the CO(3-2) emission is consistent with arising from an axisymmetric rotating disk, with an exponential profile, a rotation velocity of $v_{rot}=328\pm7$ km s$^{-1}$, and a velocity dispersion of $\sigma_{v}=62\pm7$ km s$^{-1}$. The high value of the ratio $v_{rot}/\sigma_v$, $\approx5.3$, implies that DLA-B1228g is a rotation-dominated cold disk galaxy, the second case of a high-$z$ HI-absorption-selected galaxy identified with a cold rotating disk. We obtain a dynamical mass of $M_{dyn}= (1.5\pm0.1)\times10^{11}~M_\odot$, similar to the molecular gas mass of $\approx10^{11} M_\odot$ inferred from earlier CO(1-0) studies; this implies that the galaxy is baryon-dominated in its inner regions.