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

K. E. Heintz; G. Björnsson; M. Neeleman; L. Christensen; J. P. U. Fynbo; P. Jakobsson; J. -K. Krogager; T. Laskar; C. Ledoux; G. Magdis; P. Møller; P. Noterdaeme; P. Schady; A. de Ugarte Postigo; F. Valentino; D. Watson

doi:10.1093/mnras/stab2123

paper

GRB host galaxies with strong H $_{2}$ absorption: CO-dark molecular gas at the peak of cosmic star formation

arXiv:2108.00714 · doi:10.1093/mnras/stab2123

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 ( $α_{CO}$ ). We find, $M_{mol} < 3.5 \times 1 0^{10} M_{⊙}$ (GRB\,080607), $M_{mol} < 4.7 \times 1 0^{11} M_{⊙}$ (GRB\,120815A), and $M_{mol} < 8.9 \times 1 0^{11} M_{⊙}$ (GRB\,181020A). The high limits on the molecular gas mass for the latter two cases are a consequence of their low stellar masses $M_{⋆}$ ( $M_{⋆} ≲ 1 0^{8} M_{⊙}$ ) and low gas-phase metallicities ( $Z \sim 0.03 Z_{⊙}$ ). 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-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 $M_{⋆} < 1 0^{10} M_{⊙}$ . 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.

arXiv comment7 pages, 5 Figs, accepted for publication in MNRAS

GRB host galaxies with strong H2​ absorption: CO-dark molecular gas at the peak of cosmic star formation

Abstract

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GRB host galaxies with strong H $_{2}$ absorption: CO-dark molecular gas at the peak of cosmic star formation