THE FIRST POSITIVE DETECTION OF MOLECULAR GAS IN A GRB HOST GALAXY

We report on strong H(2) 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 Hi column density (N(Hi) = 10(22.70 +/- 0.15) cm(-2)) 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(16.5 +/- 0.3) cm(-2) and T(ex)(CO) > 100 K. We argue that the high excitation temperature results from UV pumping of the CO gas by the GRB afterglow. Similarly, we observe H(2) absorption via the Lyman-Werner bands and estimate N(H2) = 10(21.2 +/- 0.2) cm(-2) with T(ex)(H2) = 10-300 K. The afterglow photometry suggests an extinction law with R(V) approximate to 4 and A(V) approximate to 3.2 mag and requires the presence of a modest 2175 angstrom bump. Additionally, modeling of the Swift XRT X-ray spectrum confirms a large column density with N(H) = 10(22.58 +/- 0.04) cm(-2). Remarkably, this molecular gas has extinction properties, metallicity, and a CO/H(2) 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.