We study the physical conditions in damped Lyman- \alpha systems ( DLAs ) , using a sample of 33 systems toward 26 QSOs acquired for a recently completed survey of H _ { 2 } by Ledoux et al .
( 2003 ) .
We use the column densities of H _ { 2 } in different rotational levels , together with those of C i , C i ^ { * } , C i ^ { ** } , C ii ^ { * } and singly ionized atomic species to discuss the kinetic temperature , the density of hydrogen and the electronic density in the gas together with the ambient UV radiation field .
Detailed comparisons are made between the observed properties in DLAs , the interstellar medium ( ISM ) of the Galaxy , the large and small Magellanic clouds ( LMC and SMC ) .

The mean kinetic temperature of the gas corresponding to DLA subcomponents in which H _ { 2 } absorption line is detected , derived from the ortho-to-para ratio ( 153 \pm 78 K ) , is higher than that measured in the ISM ( 77 \pm 17 K ) and the Magellanic clouds ( 82 \pm 21 K ) .
Typical pressure in these components ( corresponding to T = 100 - 300 K and n _ { H } = 10 - 200 cm ^ { -3 } ) , measured using C i fine-structure excitation , are higher than what is measured along ISM sightlines .
This is consistent with the corresponding higher values for N ( H _ { 2 } , J=2 ) / N ( H _ { 2 } , J=0 ) seen in DLAs .
From the column densities of the high-J rotational levels , we derive that the typical radiation field in the H _ { 2 } bearing components is of the order of or slightly higher than the mean UV field in the Galactic ISM .
Determination of electron density in the gas with H _ { 2 } and C i show the ionization rate is similar to that of a cold neutral medium ( CNM ) in a moderate radiation field .
This , together with the fact that we see H _ { 2 } in 13-20 % of the DLAs , can be used to conclude that DLAs at z > 1.9 could contribute as much as 50 % star formation rate density seen in Lyman break galaxies ( LBGs ) .

C ii ^ { * } absorption line is detected in all the components where H _ { 2 } absorption line is seen .
The excitation of C ii in these systems is consistent with the physical parameters derived from the excitation of H _ { 2 } and C i .
We detect C ii ^ { * } in about 50 % of the DLAs and therefore in a considerable fraction of DLAs that do not show H _ { 2 } .
In part of the later systems , physical conditions could be similar to that in the CNM gas of the Galaxy .
However , the absence of C i absorption line and the presence of Al iii absorption lines with a profile similar to the profiles of singly ionized species suggest an appreciable contribution from warm ( WNM ) and/or partially ionized gas .
The absence of H _ { 2 } , for the level of metallicity and dust depletion seen in these systems , are consistent with low densities ( i.e n _ { H } \leq 1 cm ^ { -3 } ) for a radiation field similar to the mean Galactic UV field .