A Massive, Dusty, HI Absorption-Selected Galaxy at z ≈ 2.46 Identified in a CO Emission Survey

Abstract

We report a NOrthern Extended Millimeter Array (NOEMA) and Atacama Large Millimeter/submillimeter Array search for redshifted CO emission from the galaxies associated with seven high-metallicity ([M/H] ≥ -1.03) damped Lyα absorbers (DLAs) at z ≈ 1.64-2.51. Our observations yielded one new detection of CO(3-2) emission from a galaxy at z = 2.4604 using NOEMA, associated with the z = 2.4628 DLA toward QSO B0201+365. Including previous searches, our search results in detection rates of CO emission of $\approx {56}_{-24}^{+38}$ % and $\approx {11}_{-9}^{+26}$ %, respectively, in the fields of DLAs with [M/H] > -0.3 and [M/H] < -0.3. Further, the HI-selected galaxies associated with five DLAs with [M/H] > -0.3 all have high molecular gas masses, ≳5 × 10¹⁰ M _⊙. This indicates that the highest-metallicity DLAs at z ≈ 2 are associated with the most massive galaxies. The newly identified z ≈ 2.4604 HI-selected galaxy, DLA0201+365g, has an impact parameter of ≈7 kpc to the QSO sightline, and an implied molecular gas mass of (5.04 ± 0.78) × 10¹⁰ × (α _CO/4.36) × (r ₃₁/0.55) M _⊙. Archival Hubble Space Telescope Wide Field and Planetary Camera 2 imaging covering the rest-frame near-ultraviolet (NUV) and far-ultraviolet (FUV) emission from this galaxy yield nondetections of rest-frame NUV and FUV emission, and a 5σ upper limit of 2.3 M _⊙ yr^-1 on the unobscured star formation rate (SFR). The low NUV-based SFR estimate, despite the very high molecular gas mass, indicates that DLA0201+365g either is a very dusty galaxy, or has a molecular gas depletion time that is around 2 orders of magnitude larger than that of star-forming galaxies at similar redshifts.