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Title:
The UCSD HIRES/Keck I Damped Lyα Abundance Database. III. An Empirical Study of Photoionization in the Damped Lyα System toward GB 1759+7539
Authors:
Prochaska, Jason X.; Howk, J. Christopher; O'Meara, John M.; Tytler, David; Wolfe, Arthur M.; Kirkman, David; Lubin, Dan; Suzuki, Nao
Affiliation:
AA(The Observatories of the Carnegie Institute of Washington, 813 Santa Barbara Street, Pasadena, CA 91101; ), AB(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218; ), AC(See http://kingpin.ucsd.edu/~hiresdla.), AD(See http://kingpin.ucsd.edu/~hiresdla.), AE(Department of Physics and Center for Astrophysics and Space Sciences, University of California at San Diego, C-0424, La Jolla, CA 92093; , , , , , ; See http://kingpin.ucsd.edu/~hiresdla.), AF(Department of Physics and Center for Astrophysics and Space Sciences, University of California at San Diego, C-0424, La Jolla, CA 92093; , , , , , ; See http://kingpin.ucsd.edu/~hiresdla.), AG(Department of Physics and Center for Astrophysics and Space Sciences, University of California at San Diego, C-0424, La Jolla, CA 92093; , , , , , ), AH(Department of Physics and Center for Astrophysics and Space Sciences, University of California at San Diego, C-0424, La Jolla, CA 92093; , , , , , )
Publication:
The Astrophysical Journal, Volume 571, Issue 2, pp. 693-711. (ApJ Homepage)
Publication Date:
06/2002
Origin:
UCP
ApJ Keywords:
Galaxies: Abundances, Galaxies: Evolution, Nuclear Reactions, Nucleosynthesis, Abundances, Galaxies: Quasars: Absorption Lines
DOI:
10.1086/340066
Bibliographic Code:
2002ApJ...571..693P

Abstract

We investigate the ionization state of the damped Lyα system at z=2.62 toward GB 1759+7539 through an analysis of ionic ratios sensitive to photoionization: Ar0/S+, Fe++/Fe+, N+/N0, and Al++/Al+. Approximately half of the metals arise in a mostly neutral velocity component with H I/H>0.9, based on Fe++/Fe+<0.013. In contrast, the remaining half exhibit Fe++/Fe+~0.3, indicative of a partially ionized medium with H I/H~0.5. These conclusions are supported by the observed N+/N0, Al++/Al+, and Ar0/Si+ ratios. We assess ionization corrections for the observed column densities through photoionization models derived from the CLOUDY software package. In the neutral gas, the ionization corrections are negligible, except for Ar0. However, for the partially ionized gas, element abundance ratios differ from the ionic ratios by 0.1-0.3 dex for (Si+, S+, Ni+, Al+)/Fe+ ratios and more for (N0, Ar0)/Fe+. Independent of the shape of the photoionizing spectrum and assumptions of the number of ionization phases, these ionization corrections have minimal impact (<~0.1 dex) on the total metallicity inferred for this damped Lyα system. Measurements of the relative elemental abundances of the partially ionized gas, however, have a greater than ~0.15 dex uncertainty, which hides the effects of nucleosynthesis and differential dust depletion. We caution the reader that this damped system is unusual for a number of reasons (e.g., a very low Ar0/S+ ratio), and we believe its ionization properties are special but not unique. Nevertheless, it clearly shows the value of examining photoionization diagnostics such as Fe++/Fe+ in a larger sample of damped systems. Visiting Astronomer, W. M. Keck Telescope. The Keck Observatory is a joint facility of the University of California and the California Institute of Technology.
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