Prochaska 2008 GRB 080607的宿主星系的分子气体

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第一个看到分子气体的伽玛暴宿主星系

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Title:		The First Positive Detection of Molecular Gas in a GRB Host Galaxy
Authors:		Prochaska, J. X.; Sheffer, Y.; Perley, D. A.; Bloom, J. S.; Lopez, L. A.; Dessauges-Zavadsky, M.; Chen, H.-W.; Filippenko, A. V.; Ganeshalingam, M.; Li, W.; Miller, A. A.; Starr, D.
Publication:		The Astrophysical Journal Letters, Volume 691, Issue 1, pp. L27-L32 (2009). (ApJ Homepage)
Publication Date:		01/2009
Origin:		IOP
Keywords:		gamma rays: bursts, ISM: molecules
DOI:		10.1088/0004-637X/691/1/L27
Bibliographic Code:		2009ApJ...691L..27P

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\,\\scriptsize {\sc 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±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 \pm 0.2} \,cm^{-2} with T_ex^H_2 = 10-300 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 Å 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/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.

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