A massive HI-absorption-selected galaxy at $z\approx 2.356$

Abstract

We use the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA) to detect $\mathrm{CO}(1-0)$, $\mathrm{CO}(3-2)$, and rest-frame 349-GHz continuum emission from an HI-selected galaxy, DLA1020+2733g, at $z\approx 2.3568$ in the field of the $z=2.3553$ damped Lyman-$\hat{I}\pm$ absorber (DLA) towards QSO J1020+2733. The VLA $\mathrm{CO}(1-0)$ detection yields a molecular gas mass of $(2.84\pm 0.42)\times 10^{11}\times (\hat{I}{\pm }_{\mathrm{CO}}/4.36){\mathrm{M}}_{\odot }$, the largest ever measured in an HI-selected galaxy. The DLA metallicity is $+0.28\pm 0.16$, from the ZnII$\lambda 2026$Ã absorption line detected in a Keck Echellette Spectrograph and Imager spectrum. This continues the trend of high-metallicity DLAs being frequently associated with massive galaxies. We obtain a star-formation rate (SFR) of $\lesssim 400$M${}_{\odot }$yr${}^{-1}$ from the rest-frame 349-GHz continuum emission, and a relatively long molecular gas depletion timescale of $\gtrsim 0.6$Gyr. The excitation of the J=3 rotational level is sub-thermal, with $r_{31}\equiv L_{\mathrm{CO}(3-2)}^{\prime }/L_{\mathrm{CO}(1-0)}^{\prime }=0.513\pm 0.081$, suggesting that DLA1020+2733g has a low SFR surface density. The large velocity spread of the CO lines, $\approx 500\mathrm{km}{\mathrm{s}}^{-1}$, and the long molecular gas depletion timescale suggest that DLA1020+2733g is likely to be a cold rotating-disk galaxy.

Accepted for publication in ApJ Letters; 11 pages, 6 figures, and 2 Tables