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 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 ($\hat{I}{\pm }_{\mathrm{CO}}$). We find, $M_{\mathrm{mol}}<3.5\times 10^{10}{M}_{\odot }$ (GRB\,080607), $M_{\mathrm{mol}}<4.7\times 10^{11}{M}_{\odot }$ (GRB\,120815A), and $M_{\mathrm{mol}}<8.9\times 10^{11}{M}_{\odot }$ (GRB\,181020A). The high limits on the molecular gas mass for the latter two cases are a consequence of their low stellar masses $M_{\star }$ ($M_{\star }\lesssim 10^8{M}_{\odot }$) and low gas-phase metallicities ($Z\sim 0.03Z_{\odot }$). The limit on the $M_{\mathrm{mol}}/M_{\star }$ 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 $\hat{I}{\pm }_{\mathrm{CO}}$, will severely underestimate the actual molecular gas mass for all galaxies at $z>1$ with $M_{\star }<10^{10}{M}_{\odot }$. To better quantify this we develop a simple approach to estimate the relevant $\hat{I}{\pm }_{\mathrm{CO}}$ 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.

7 pages, 5 Figs, accepted for publication in MNRAS