2021MNRAS.507.1434H

We present a pilot search of CO emission in three H₂-absorbing, long-duration gamma-ray burst (GRB) host galaxies at z ∼ 2-3. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to target the CO(3 - 2) emission line and report non-detections for all three hosts. These are used to place limits on the host molecular gas masses, assuming a metallicity-dependent CO-to-H₂ conversion factor (α_CO). We find, M_mol < 3.5×10¹⁰ M_☉ (GRB 080607), M_mol < 4.7 ×10¹¹ M_☉ (GRB 120815A), and M_mol < 8.9×10¹¹ 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_*≲_10⁸ 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-H₂ 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_*<10¹⁰ 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.