The First Positive Detection of Molecular Gas in a GRB Host Galaxy
Abstract
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 (N_H I = 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) = 1016.5±0.3 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 N_H_2= 10^{21.2 ± 0.2} cm^{-2} with T_ex^H_2 = 10-300 K. The afterglow photometry suggests an extinction law with RV ≈ 4 and AV ≈ 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 = 1022.58±0.04 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2009
- DOI:
- 10.1088/0004-637X/691/1/L27
- arXiv:
- arXiv:0901.0556
- Bibcode:
- 2009ApJ...691L..27P
- Keywords:
-
- gamma rays: bursts;
- ISM: molecules;
- Astrophysics - High Energy Astrophysical Phenomena;
- Astrophysics - Cosmology and Extragalactic Astrophysics
- E-Print:
- 6 pages, 4 figures, 1 table. Accepted to ApJL on December 3, 2008