High Molecular Gas Masses in Absorption-selected Galaxies at z ≈ 2

Abstract

We have used the Atacama Large Millimeter/submillimeter Array (ALMA) to carry out a search for CO (3−2) or (4−3) emission from the fields of 12 high-metallicity ([M/H]~≥−0.72\,dex) damped Lyman-α absorbers (DLAs) at z≈1.7−2.6. We detected CO emission from galaxies in the fields of five DLAs (two of which have been reported earlier), obtaining high molecular gas masses, M_mol≈(1.3−20.7)×(α_CO/4.36)×10¹⁰M_⊙. The impact parameters of the CO emitters to the QSO sightline lie in the range b≈5.6−100~kpc, with the three new CO detections having b≲15~kpc. The highest CO line luminosities and inferred molecular gas masses are associated with the highest-metallicity DLAs, with [M/H]~≳−0.3\,dex. The high inferred molecular gas masses may be explained by a combination of a stellar mass-metallicity relation and a high molecular gas-to-stars mass ratio in high-redshift galaxies; the DLA galaxies identified by our CO searches have properties consistent with those of emission-selected samples. None of the DLA galaxies detected in CO emission were identified in earlier optical or near-IR searches and vice-versa; DLA galaxies earlier identified in optical/near-IR searches were not detected in CO emission. The high ALMA CO and C[{\sc ii}]~158μm detection rate in high-z, high-metallicity DLA galaxies has revolutionized the field, allowing the identification of dusty, massive galaxies associated with high-z DLAs. The H{\sc i}-absorption criterion identifying DLAs selects the entire high-z galaxy population, including dusty and UV-bright galaxies, in a wide range of environments.