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

TL;DR

This study investigates molecular gas in high-redshift GRB host galaxies using ALMA, finding that low metallicity leads to high CO-to-H2 conversion factors, making many galaxies CO-dark and challenging to detect in emission.

Contribution

The paper introduces a method to estimate the CO-to-H2 conversion factor based on galaxy redshift and stellar mass, highlighting the prevalence of CO-dark molecular gas in low-metallicity, high-redshift galaxies.

Findings

Non-detections of CO emission in three GRB host galaxies.

High molecular gas mass limits due to low metallicity and stellar mass.

Canonical CO conversion factors underestimate molecular gas in low-mass, high-redshift galaxies.

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 ($\alpha_{\rm CO}$). We find, $M_{\rm mol} < 3.5\times 10^{10}\,M_{\odot}$ (GRB\,080607), $M_{\rm mol} < 4.7\times 10^{11}\,M_{\odot}$ (GRB\,120815A), and $M_{\rm mol} < 8.9\times 10^{11}\,M_{\odot}$ (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 ($Z\sim 0.03\,Z_{\odot}$). The limit on the $M_{\rm mol}/M_\star$ 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 $\alpha_{\rm CO}$, 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 $\alpha_{\rm CO}$ 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.