Understanding Physical Conditions in High-redshift Galaxies Through C I Fine Structure Lines: Data and Methodology

• Published in: The Astrophysical journal Vol. 722; no. 1; pp. 460 - 490
• Main Authors: Jorgenson, Regina A, Wolfe, Arthur M, Prochaska, J. Xavier
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 10.10.2010; IOP
• Subjects: Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CHEMICAL REACTIONS; COSMIC RADIO SOURCES; DECOMPOSITION; Earth, ocean, space; ELEMENTS; EVOLUTION; Exact sciences and technology; GALACTIC EVOLUTION; GALAXIES; HYDROGEN; IONIZATION; MILKY WAY; NONMETALS; PHOTOCHEMICAL REACTIONS; PHOTOLYSIS; QUASARS; RED SHIFT; STAR EVOLUTION; STARS; Galaxy evolution; Quasars; Intergalactic matter; Red shift; Galaxies; Molecular clouds; galaxies: evolution; quasars: absorption lines; Ionization equilibrium; intergalactic medium; Photodissociation; Column density; Milky Way; Precursor; Fine structure; Absorption line; Star formation; Standard deviation; Hydrogen molecules
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/722/1/460

We probe the physical conditions in high-redshift galaxies, specifically, the damped Ly{alpha} systems (DLAs) using neutral carbon (C I) fine structure lines and molecular hydrogen (H{sub 2}). We report five new detections of C I and analyze the C I in an additional two DLAs with previously published data. We also present one new detection of H{sub 2} in a DLA. We present a new method of analysis that simultaneously constrains both the volume density and the temperature of the gas, as opposed to previous studies that a priori assumed a gas temperature. We use only the column density of C I measured in the fine structure states and the assumption of ionization equilibrium in order to constrain the physical conditions in the gas. We present a sample of 11 C I velocity components in six DLAs and compare their properties to those derived by the global C II* technique. The resulting median values for this sample are (n(H I)) = 69 cm{sup -3}, (T) = 50 K, and (log(P/k)) = 3.86 cm{sup -3} K, with standard deviations, {sigma}{sub n(H{sub i})} = 134 cm{sup -3}, {sigma}{sub T} = 52 K, and {sigma}{sub log(P/k)} = 3.68 cm{sup -3} K. This can be compared with the integrated median values for the same DLAs: (n(H I)) = 2.8 cm{sup -3}, (T) = 139 K, and (log(P/k)) = 2.57 cm{sup -3} K, with standard deviations {sigma}{sub n(H{sub i})} = 3.0 cm{sup -3}, {sigma}{sub T} = 43 K, and {sigma}{sub log(P/k)} = 0.22 cm{sup -3} K. Interestingly, the pressures measured in these high-redshift C I clouds are similar to those found in the Milky Way. We conclude that the C I gas is tracing a higher-density, higher-pressure region, possibly indicative of post-shock gas or a photodissociation region on the edge of a molecular cloud. We speculate that these clouds may be direct probes of the precursor sites of star formation in normal galaxies at high redshift.