A Massive H i-absorption-selected Galaxy at z ≈ 2.356

Abstract

We use the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array to detect CO(1–0), CO(3–2), and rest-frame 349 GHz continuum emission from an H i -selected galaxy, DLA1020+2733g, at z ≈ 2.3568 in the field of the z = 2.3553 damped Ly α absorber (DLA) toward QSO J1020+2733. The VLA CO(1–0) detection yields a molecular gas mass of (2.84 ± 0.42) × 10 ^11 × ( α _CO /4.36) M _⊙ , the largest ever measured in an H i -selected galaxy. The DLA metallicity is +0.28 ± 0.16, from the Zn ii λ 2026 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 ≲400 M _⊙ yr ^−1 from the rest-frame 349 GHz continuum emission and a relatively long molecular gas depletion timescale of ≳0.6 Gyr. The excitation of the J = 3 rotational level is subthermal, with ${r}_{31}\equiv L{{\prime} }_{{\rm{CO}}(3-2)}/L{{\prime} }_{{\rm{CO}}(1-0)}=0.513\pm 0.081$ , suggesting that DLA1020+2733g has a low SFR surface density. The large velocity spread of the CO lines, ≈500 km s ^−1 , and the long molecular gas depletion timescale suggest that DLA1020+2733g is likely to be a cold rotating-disk galaxy.

Keywords

• Damped Lyα systems
• Radio interferometry
• High-redshift galaxies