The first positive detection of molecular gas in a GRB host galaxy

We report on strong H₂ and CO absorption from gas within the host galaxy of gamma-ray burst (GRB) 080607. Analysis of our Keck/LRIS afterglow spectrum reveals a very large H I column density () and strong metal-line absorption at z _GRB = 3.0363 with a roughly solar metallicity. We detect a series of A - X bandheads from CO and estimate N(CO) = 10 ^16.50.3 cm^-2 and T ^CO _ex > 100 K. We argue that the high excitation temperature results from UV pumping of the CO gas by the GRB afterglow. Similarly, we observe H₂ absorption via the Lyman-Werner bands and estimate with -300 K. The afterglow photometry suggests an extinction law with R_V 4 and A_V 3.2 mag and requires the presence of a modest 2175 bump. Additionally, modeling of the Swift XRT X-ray spectrum confirms a large column density with N _H = 10^22.580.04 cm^-2. Remarkably, this molecular gas has extinction properties, metallicity, and a CO/H₂ ratio comparable to those of translucent molecular clouds of the Milky Way, suggesting that star formation at high z proceeds in similar environments as today. However, the integrated dust-to-metals ratio is sub-Galactic, suggesting the dust is primarily associated with the molecular phase while the atomic gas has a much lower dust-to-gas ratio. Sightlines like GRB 080607 serve as powerful probes of nucleosynthesis and star-forming regions in the young universe and contribute to the population of "dark" GRB afterglows.