Physical Conditions In Damped Lyman Alpha Systems

The primary goal of this thesis is to determine physical conditions in high redshift galaxies known as Damped Lymna alpha Systems. To this end, we present several key results. First, we discuss a radio-selected quasar survey that demonstrates that dust bias is likely not an issue affecting surveys for DLAs. In other words, we are seeing the true and complete population of DLAs and not missing some very high column density, dusty DLAs . Second, we use neutral carbon (CI) fine structure states to show the presence of gas with densities and pressures 1-2 orders of magnitude larger than those expected in the global DLA, i.e. the presence of knots of dense cold gas existing in the larger diffuse DLA. To solve the imbalance of pressure between these two phases, we speculate that these regions are created by shocks that provide relatively high densities that in turn allow for the existence of CI. Alternatively, these could be photodissociation regions on the edges of even more dense molecular clouds that could even be hosting star formation. This model would explain the presence of a radiation field generally above the background. We also present unambiguous evidence of the presence of cold gas in DLAs -- the discovery of narrow sub-resolution, sub-1 km s−1 CI components with thermal temperatures