The cold neutral phase of the interstellar medium in high redshift galaxies

We present the results of spectroscopic analysis of seven new H₂-bearing damped Ly- systems in redshift range z=2.5 − 3. These systems were originally selected from SDSS catalog using a direct search for H₂ and followed up with X-SHOOTER spectrograph at 8-m Very Large Telescope observatory. We measured the column densities of H_I, H₂ on various rotational levels, and metals species in different ionization stages and excitation levels. We used the rotational excitation of H₂ molecules together with the fine-structure levels of neutral carbon to constrain the physical conditions in the associated medium. We found typical values for the kinetic temperature T ∼ 80 − 120 K, hydrogen density n _H ∼ 30 − 400 cm⁻³ and UV radiation field ξ_UV ∼ 0.4 − 5 times of the Draine field. These values along with estimated thermal pressure are in agreement with expected values from the theoretical calculation of the cold neutral interstellar medium. (paper)