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 ALMA survey of molecular gas in galaxies selected for their strong HI absorption, so-called 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 spectra of the galaxy revealing [OII], Hα and [NII] 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 . Surprisingly, this HI absorption-selected galaxy has no equivalent objects in CO surveys of flux-selected samples. The galaxy falls off current scaling relations for the 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 de-coupled from the main galaxy. It is thus possible that a major star burst in the past could explain the wide CO profile as well as the low SFR. Support for this also comes from the SED 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.

ALMA observations of a metal-rich damped Ly α absorber at z = 2.5832: evidence for strong galactic winds in a galaxy group

“…The spectrum shows that the CO emission is redshifted from the centroid of the DLA at z = 2.5832 by 131 km s −1 . There is marginal evidence for a "boxy" or double-horned line profile (Davis et al 2011), similar to the CO(2−1) and CO(1−0) line profiles observed for two other, low-z absorbing galaxies (Neeleman et al 2016;Møller et al 2018). This indicates that this galaxy has some degree of a rotational support and/or emission from several sub-clumps.…”

“…The first pilot studies of DLAs with ALMA have given quite interesting results. Møller et al (2018) studied a single system at z = 0.716 and found a surprisingly large molecular mass and for that mass surprisingly low star-formation rate placing the object away from the normal galactic scaling relations for these quantities. Kanekar et al (2018) studied a sample of DLAs at similar redshifts and found a surprisingly large detection rate of CO-emission from the galaxy counterparts of the absorbers.…”

Characterizing the circum-galactic medium of damped Lyman-α absorbing galaxies

“…In addition to the optical searches such as the one described here, there are on-going campaigns detecting with ALMA molecular emission lines from absorption selected galaxies (Neeleman et al 2016(Neeleman et al , 2018Klitsch et al 2018;Møller et al 2018;Kanekar et al 2018). Combining optical and sub-mm wavelengths observations of absorber host galaxies will bring new insights into the role of molecular gas in the CGM of galaxies.…”

Multiphase circumgalactic medium probed with MUSE and ALMA

“…These molecular masses are large in comparison with the typical stellar mass of absorbing galaxies . In fact, a number of recent works have reported CO associated with quasar absorbers from ALMA observations with a high rate of incidence and derived equally high molecular masses (Neeleman et al 2016;Møller et al 2018;Neeleman et al 2018;Kanekar et al 2018;Klitsch et al 2018;D'Odorico et al 2018;Klitsch et al 2019). However, it is unclear why H i-rich absorbing galaxies should have large reservoir of H2 gas.…”