2009ApJ...704..247J

We present the first direct evidence for cold gas in a high redshift DLA galaxy. We measured several multiplets of weak neutral carbon (C I) transitions in order to perform a curve of growth analysis. A Δ χ² test constrains the best-fit Doppler parameter, b = 0.33^+0.05_–0.04 km/s and log N(C I) = 13.30±0.2/cm2. This Doppler parameter constrains the kinetic temperature of the gas to T ≤ 78 K (T ≤ 115 K, 2σ). We used the associated C I fine structure lines to constrain the volume density of the gas, n(H I) ∼40-200/cm3 (2σ), resulting in a lower limit on the cloud size of approximately 0.1-1 parsec. While it is difficult to determine the metallicity of the cold component, the absence of Cr II indicates that the cold cloud suffers a high level of dust depletion. Additionally, the large amount of Lyman and Werner-band molecular hydrogen absorption (log N(H₂)_total= 19.88/cm2, f H₂ ≥ 0.06) with an excitation temperature of T_ex= 46 K as determined by the rotational J = 0 and J = 1 states, is consistent with the presence of cold gas. We propose that this cloud may be gravitationally confined and may represent a transition gas phase from primarily neutral atomic gas, to a colder, denser molecular phase that will eventually host star formation.