THE PROPERTIES OF LYMAN LIMIT ABSORBING CLOUDS AT Z = 3 - PHYSICAL CONDITIONS IN THE EXTENDED GASEOUS HALOS OF HIGH-REDSHIFT GALAXIES

A detailed study of eight known Lyman limit absorption systems with 2.90 ≤ zabs ≤ 3.23 has been undertaken in order to provide an unbiased preliminary survey of the properties of heavy element absorption systems at very high redshift. Selected spectral regions for each QSO have been observed at 35-80 km s-1 resolution and high signal-to-noise ratio so that the column densities of H I and associated ions of heavy elements might be determined. The H I column densities are in the range 17.0 ≤ log N(H I) ≤ 19.3. The column density measurements have been used in conjunction with photoionization models in order to estimate the physical parameters of the clouds or cloud ensembles responsible for the Lyman discontinuities. Although there are many uncertainties involved in the analysis, the heavy element abundances appear to be in the range -3.0 ≤[M/H] ≤ -1.5, and the relative abundances of C, O, and Si, although uncertain, appear to be consistent with a uniform depletion relative to the solar values. If the results of the photoionization models are accurate, the particle density in the clouds is in the range 0.01 ≲ nH ≲ 0.1, and the corresponding cloud sizes and masses are inferred to be in the range 1 ≤ Dc ≤ 15 kpc and 106 ≤ Mc ≤ 109 M⊙, respectively. Such parameters would be expected on theoretical grounds for clouds that condensed in extended (≳ 100 kpc) gaseous halos during the early evolution of normal galaxies. A considerable fraction of the inferred visible mass of present-day galaxies was probably in the form of these diffuse clouds at the epoch corresponding to z ≈ 3; thus, they have probably played an important role in the evolution of galaxies with time. The possibility that the heavy element absorbing clouds and the "Lyman-α forest" clouds have a common origin (but evolve differently because of their environment) is discussed.