Ledoux, C. 2009 高红移伽玛暴余辉对DLA吸收体的限制

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Title:		Physical conditions in high-redshift GRB-DLA absorbers observed with VLT/UVES: Implications for molecular hydrogen searches
Authors:		Ledoux, C.; Vreeswijk, P. M.; Smette, A.; Fox, A. J.; Petitjean, P.; Ellison, S. L.; Fynbo, J. P. U.; Savaglio, S.
Publication:		eprint arXiv:0907.1057
Publication Date:		07/2009
Origin:		ARXIV
Keywords:		Astrophysics - Cosmology and Extragalactic Astrophysics
Comment:		16 pages, 8 figures, accepted for publication in A&A
Bibliographic Code:		2009arXiv0907.1057L

Abstract

We aim to understand the nature of the absorbing neutral gas in the galaxies hosting high-redshift long-duration GRBs and to determine their physical conditions. We report the detection of a significant number of previously unidentified allowed transition lines of Fe+, involving the fine structure of the ground term and that of other excited levels, from the zabs=3.969, log N(H0)=22.10 DLA system located in the host galaxy of GRB 050730. The time-dependent evolution of the observed Fe+ energy-level populations is modelled by assuming the excitation mechanism is fluorescence following excitation by ultraviolet photons. This UV pumping model successfully reproduces the observations, yielding a distance of the GRB-facing front of the cloud to the burst of d=440\pm 30 pc and a linear cloud size of l=520{+240}{-190} pc. We discuss these results in the context of no detections of H2 and CI lines in a sample of seven z>1.8 GRB host galaxies observed with VLT/UVES. We show that the lack of H2 can be explained by the low metallicities, [X/H]<-1, low depletion factors and, at most, moderate particle densities of the systems. This points to a picture where GRB-DLAs typically exhibiting very high H0 column densities are diffuse metal-poor atomic clouds with high kinetic temperatures, Tkin>~1000 K, and large physical extents, l>~100 pc. The properties of GRB-DLAs observed at high spectral resolution towards bright GRB afterglows differ markedly from the high metal and dust contents of GRB-DLAs observed at lower resolution. This difference likely results from the effect of a bias, against systems of high metallicity and/or close to the GRB, due to dust obscuration in the magnitude-limited GRB afterglow samples observed with high-resolution spectrographs.

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