GRB host galaxies with strong H₂absorption: CO-dark molecular gas at the peak of cosmic star formation

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 nondetections 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 (αCO). We find, Mmol < 3.5 × 1010M⊙ (GRB 080607), Mmol < 4.7 × 1011M⊙ (GRB 120815A), and Mmol < 8.9 × 1011M⊙ (GRB 181020A). The high limits on themolecular gasmass for the latter two cases are a consequence of their low stellar masses M∗ (M∗ ≲ 108M⊙) and low gas-phase metallicities (Z ∼ 0.03Z⊙). The limit on the Mmol/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 αCO will severely underestimate the actual molecular gasmass for all galaxies at z>1 with M∗ < 1010M⊙. To better quantify this we develop a simple approach to estimate the relevant αCO 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.