A new method is used to measure the physical conditions of the gas in damped Lyman- \alpha systems ( DLAs ) .
Using high resolution absorption spectra of a sample of 80 DLAs , we are able to measure the ratio of the upper and lower fine-structure levels of the ground state of C ^ { + } and Si ^ { + } .
These ratios are determined solely by the physical conditions of the gas .
We explore the allowed physical parameter space using a Monte Carlo Markov Chain method to constrain simultaneously the temperature , neutral hydrogen density , and electron density of each DLA .
The results indicate that at least 5 % of all DLAs have the bulk of their gas in a dense , cold phase with typical densities of \sim 100 cm ^ { -3 } and temperatures below 500 K. We further find that the typical pressure of DLAs in our sample is \log ( P / k _ { B } ) = 3.4 [ K cm ^ { -3 } ] , which is comparable to the pressure of the local interstellar medium ( ISM ) , and that the components containing the bulk of the neutral gas can be quite small with absorption sizes as small as a few parsec .
We show that the majority of the systems are consistent with having densities significantly higher than expected from a purely canonical WNM , indicating that significant quantities of dense gas ( i.e .
n _ { H } > 0.1 cm ^ { -3 } ) are required to match observations .
Finally , we identify 8 systems with positive detections of Si ii ^ { * } .
These systems have pressures ( P / k _ { B } ) in excess of 20000 K cm ^ { -3 } , which suggest that these systems tag a highly turbulent ISM in young , star-forming galaxies .