Spectroscopic Observations of High Redshift Qsos: Galaxies and the Intergalactic Medium at Early Epochs.

The thesis consists primarily of 4 parts, which are separate but related insofar as each makes use of spectra of high redshift QSOs in order to investigate the properties of intervening gaseous material: (I) Wide-slit spectrophotometry of a sample of very high redshift QSOs is used in combination with high resolution observations of QSO Lyman alpha forests to obtain a new upper limit on the denisty of generally distributed intergalactic neutral hydrogen (the Gunn-Peterson limit). Implications for the nature of the intergalactic medium are discussed. (II) Observations of a number of high redshift QSOs close to each other on the plane of the sky are presented; based on the occurrence of heavy element absorption systems over a narrow range of redshifts in each of the spectra, evidence for the presence of a huge, elongated supercluster of galaxies at z = 2 is discussed. (III) Results of a survey for heavy element absorption systems at the largest observable redshift (2.7 < z < 3.8) are presented. The data show very clear evolution of the heavy element absorbers with time over the redshift range 1.5 < z < 3.5. The changes are interpreted as a gradual evolution of the heavy element abundances in gas associated with sites where galaxies have formed. The implications for galaxy formation and evolution are discussed. (IV) Moderate dispersion spectra of a sample of Lyman limit absorption systems at z ~ 3 have been obtained in order to determine the heavy element abundances and other physical parameters of the gas clouds responsible for the high redshift, heavy-element absorption systems in the spectra of QSOs. The abundances of C, O, and Si are in general between 0.001 and 0.01 of the solar abundances, comparable to the most metal-poor stars in the halo of the Galaxy. The clouds have sizes in the range 1-15 kpc and masses in the range 10^6 - 10^9 M_odot . The clouds are discussed within the context of the hypothesis that the heavy element absorption systems are associated with the extended halos of high redshift galaxies. Diffuse gas in the form of such clouds may account for a significant fraction of the baryonic material in the universe, and it has probably played an important role in the evolution of galaxies over time.