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 H i column density ( N _ { H~ { } I } = 10 ^ { 22.70 \pm 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 \pm 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 _ { H _ { 2 } } = 10 ^ { 21.2 \pm 0.2 } { cm ^ { -2 } } with T _ { ex } ^ { H _ { 2 } } = 10 –300 K. The afterglow photometry suggests an extinction law with R _ { V } \approx 4 and A _ { V } \approx 3.2 mag and requires the presence of a modest 2175 Å bump .
Additionally , modeling of the Swift /XRT X-ray spectrum confirms a large column density with N _ { H } = 10 ^ { 22.58 \pm 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 .