2010ApJ...722..460J

We probe the physical conditions in high-redshift galaxies, specifically, the damped Lyα systems (DLAs) using neutral carbon (C I) fine structure lines and molecular hydrogen (H₂). 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 H₂ 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/cm3, 〈T〉 = 50 K, and 〈log(P/k)〉 = 3.86/cm3 K, with standard deviations, σ{_{n(H i)}} = 134/cm3, σ_T= 52 K, and σ_log(P/k)= 3.68/cm3 K. This can be compared with the integrated median values for the same DLAs: 〈n(H I)〉 = 2.8/cm3, 〈T〉 = 139 K, and 〈log(P/k)〉 = 2.57/cm3 K, with standard deviations σ{_{n(H i)}} = 3.0/cm3, σ_T= 43 K, and σ_log(P/k)= 0.22/cm3 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.