GRB host galaxies with strong H$_2$ absorption: CO-dark molecular gas at the peak of cosmic star formation

Abstract

We present a pilot search of CO emission in three H 2 -absorbing, long-duration gamma-ray burst (GRB) host galaxies at z ∼ 2−3. We used the Atacama Large Millimeter/sub-millimeter Array (ALMA) to target the CO(3 − 2) emission line and report nondetections for all three hosts. These are used to place limits on the host molecular gas masses, assuming a metallicity-dependent CO-to-H 2 conversion factor (α CO ). We find, M mol < 3.5 × 10 10 M (GRB 080607), M mol < 4.7 × 10 11 M (GRB 120815A), and M mol < 8.9 × 10 11 M (GRB 181020A). The high limits on the molecular gas mass for the latter two cases are a consequence of their low stellar masses M (M 10 8 M ) and low gas-phase metallicities (Z ∼ 0.03 Z ). The limit on the M mol /M ratio derived for GRB 080607, however, is consistent with the average population of star-forming galaxies at similar redshifts and stellar masses. We discuss the broader implications for a metallicity-dependent CO-to-H 2 conversion factor, and demonstrate that the canonical Galactic α CO , will severely underestimate the actual molecular gas mass for all galaxies at z > 1 with M < 10 10 M . To better quantify this we develop a simple approach to estimate the relevant α CO factor based only on the redshift and stellar mass of individual galaxies. The elevated conversion factors will make these galaxies appear CO-"dark" and difficult to detect in emission, as is the case for the majority of GRB hosts. GRB spectroscopy thus offers a complementary approach to identify low-metallicity, star-forming galaxies with abundant molecular gas reservoirs at high redshifts that are otherwise missed by current ALMA surveys.