GRB host galaxies with strong H2 absorption: CO-dark molecular gas at the peak of cosmic star formation
Résumé
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/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-H_2 conversion factor (α_CO). We find, |$M_{\rm mol} \lt 3.5\times 10^{10}\, M_{\odot }$| (GRB 080607), |$M_{\rm mol} \lt 4.7\times 10^{11}\, M_{\odot }$| (GRB 120815A), and |$M_{\rm mol} \lt 8.9\times 10^{11}\, M_{\odot }$| (GRB 181020A). The high limits on the molecular gas mass for the latter two cases are a consequence of their low stellar masses M_⋆ (|$M_\star \lesssim 10^{8}\, M_{\odot }$|) and low gas-phase metallicities (|$Z\sim 0.03\, Z_{\odot }$|). 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 |$M_\star \lt 10^{10}\, M_\odot$|. 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.
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