Using a novel method, I measure the physical conditions of atomic gas in Damped Ly-α Systems (DLAs) at high redshift. To be specific, the density ratios between upper and lower fine structure states of singly ionized carbon and silicon are solely determined by the physical conditions of the gas. By measuring this ratio for a sample of 80 DLAs, I am able to put constraints on the temperature, electron density, and neutral hydrogen density of the gas in these DLAs. To provide and estimate of the uncertainty on these measurements, I apply a Monte Carlo Markov Chain method to sample the posterior distribution of these parameters. One such run is shown in the figure above. The red contours mark the 1-σ distribution after applying the requirement that the gas is mostly neutral, which is a reasonable assumption for DLAs.

The results indicate that at least 5 % of the DLA population have the bulk of their gas in a dense, cold phase with typical physical conditions that are quite similar to the local cold neutral medium observed in our Milky Way (T < 500 K, nH~100 cm-3). In addition, the majority of the DLAs have gas at relatively high densities, significantly higher than expected from a canonical warm neutral medium (nH > 0.1 cm-3). Typical pressures that we find are similar to the pressures in the local interstellar medium (log(P/kb = 3.4 [K cm-3]). By equating the density with the H I column density of the absorption, we are able to provide an estimate of the characteristic absorption length, which can be as small as a few parsec for some of these systems. Finally we have identified 8 intervening DLAs with positive Si II* detections, the largest sample to date. These systems have systematically higher pressures (P/Kb > 20000 K cm-3, suggesting these systems probe the turbulent ISM of young star-forming galaxies.