Jansky Very Large Array detections of CO(1-0) emission in HI-absorption-selected galaxies at $z\gtrsim 2$

Abstract

We report a Karl G. Jansky Very Large Array search for redshifted CO(1-0) emission from three HI-absorption-selected galaxies at $z\approx 2$, identified earlier in their CO(3-2) or CO(4-3) emission. We detect CO(1-0) emission from DLA B1228-113 at $z\approx 2.1933$ and DLA J0918+1636 at $z\approx 2.5848$; these are the first detections of CO(1-0) emission in high-$z$ HI-selected galaxies. We obtain high molecular gas masses, ${\mathrm{M}}_{\mathrm{mol}}\approx 10^{11}\times (\hat{\mathrm{I}}{\pm }_{\mathrm{CO}}/4.36){\mathrm{M}}_{\odot }$, for the two objects with CO(1-0) detections, which are a factor of $\approx 1.5-2$ lower than earlier estimates. We determine the excitation of the mid$-J$ CO rotational levels relative to the $J=1$ level, r${}_{J1}$, in HI-selected galaxies for the first time, obtaining r${}_{31}=1.00\pm 0.20$ and r${}_{41}=1.03\pm 0.23$ for DLA J0918+1636, and r${}_{31}=0.86\pm 0.21$ for DLA B1228-113. These values are consistent with thermal excitation of the $J=3,4$ levels. The excitation of the $J=3$ level in the HI-selected galaxies is similar to that seen in massive main-sequence and sub-mm galaxies at $z\gtrsim 2$, but higher than that in main-sequence galaxies at $z\approx 1.5$; the higher excitation of the galaxies at $z\gtrsim 2$ is likely to be due to their higher star-formation rate (SFR) surface density. We use Hubble Space Telescope Wide Field Camera 3 imaging to detect the rest-frame near-ultraviolet emission of DLA B1228-113, obtaining an NUV SFR of $4.44\pm 0.47$ M${}_{\odot }$ yr${}^{-1}$, significantly lower than that obtained from the total infrared luminosity, indicating significant dust extinction in the $z\approx 2.1933$ galaxy.

9 pages, 4 figures, and 2 tables. Accepted for publication in ApJL