GRB host galaxies with strong H2absorption: 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

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Pages (from-to)1434-1440
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date29 Jul 2021
Publication statusPublished - 1 Oct 2021


  • galaxies: high-redshift
  • galaxies: ISM
  • galaxies: star formation
  • gamma-ray burst: general
  • ISM: molecules

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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