High Redshift Lyman Limit and Damped Lyman-Alpha Absorbers

We have obtained high signal:to:noise optical spectroscopy at 5\AA
resolution of 27 quasars from the APM z$>$4 quasar survey. The spectra have been analyzed to create new samples of high redshift Lyman-limit and damped Lyman-$\alpha$ absorbers. These data have been combined with published data sets in a study of the redshift evolution and the column density distribution function for absorbers with $\log$N(HI)$\ge 17.5$, over the redshift range 0.01 $<$ z $<$ 5. The main results are: \begin{itemize} \item Lyman limit systems: The data are well fit by a power law $N(z)=N_0(1+z{)}^{\gamma }$ for the number density per unit redshift. For the first time intrinsic evolution is detected in the product of the absorption cross-section and comoving spatial number density for an $\mathrm{\Omega }=1$ Universe. We find $\gamma =1.55$ ($\gamma = 0.5$fornoevolution)and$N_0 = 0.27$with$>$99.7\mathrm{\%}confidencelimitsfor$\gamma$ of 0.82 & 2.37. \item Damped \lya systems: The APM QSOs provide a substantial increase in the redshift path available for damped surveys for $z>3$. Eleven candidate and three confirmed damped Ly$\alpha$ absorption systems, have been identified in the APM QSO spectra covering the redshift range $2.8\le z\le 4.4$ (11 with $z>3.5$). Combining the APM survey confirmed and candidate damped \lya absorbers with previous surveys, we find evidence for a turnover at z$\sim$3 or a flattening at z$\sim$2 in the cosmological mass density of neutral gas, ${\mathrm{\Omega }}_g$. \end{itemize} The Lyman limit survey results are published in Storrie-Lombardi, et~al., 1994, ApJ, 427, L13. Here we describe the results for the DLA population of absorbers.