ALMA + VLT observations of a damped Lyman-α absorbing galaxy: massive, wide CO emission, gas-rich but with very low SFR

Abstract

We are undertaking an Atacama Large Millimeter Array survey of molecular gas in galaxies selected for their strong H I absorption, so-called damped Lyα absorber (DLA)/sub-DLA galaxies. Here, we report CO(2–1) detection from a DLA galaxy at z = 0.716. We also present optical and near-infrared (NIR) spectra of the galaxy revealing [O II], Hα, and [N II] emission lines shifted by ∼170 km s^-1 relative to the DLA, and providing an oxygen abundance 3.2 times solar, similar to the absorption metallicity. We report low unobscured SFR∼1 M_⊙ yr^-1 given the large reservoir of molecular gas, and also modest obscured SFR =4.5+4.4−2.6 M_⊙ yr^-1 based on far-IR and sub-millimetre data. We determine mass components of the galaxy: log[M_∗/M_⊙]=10.80^+0.07−_0.14⁠, log[Mmol-gas/M_⊙] = 10.37 ± 0.04, and log[Mi_dust/M_⊙]=8.45^+0.10_−0.30⁠. Surprisingly, this H I absorption-selected galaxy has no equivalent objects in CO surveys of flux-selected samples. The galaxy falls off current scaling relations for the star formation rate (SFR) to molecular gas mass and CO Tully–Fisher relation. Detailed comparison of kinematical components of the absorbing, ionized, and molecular gas, combined with their spatial distribution, suggests that part of the CO gas is both kinematically and spatially decoupled from the main galaxy. It is thus possible that a major starburst in the past could explain the wide CO profile as well as the low SFR. Support for this also comes from the spectral energy distribution favouring an instantaneous burst of age ≈0.5 Gyr. Our survey will establish whether flux-selected surveys of molecular gas are missing a key stage in the evolution of galaxies and their conversion of gas to stars.