The Hubble Space Telescope Quasar Absorption Line Key Project. VII. Absorption Systems at Z abs <= 1.3

We present evidence that clumps of Lyα lines are physically associated with about half of the extensive metal-line systems (absorption systems with four or more observed metal-line species) found in this paper, demonstrate that all four Lyman-limit systems discussed here correspond to extensive metal-line absorption systems, and present an extraordinary pair of extensive metal-line absorption systems within 2000 km s^-1^ of each other at z = 0.95 that are probably an early manifestation of large-scale structure. These results are obtained using ultraviolet spectra, taken with the higher resolution gratings of the Faint Object Spectrograph of the Hubble Space Telescope (HST) for four quasars with emission-line redshifts between 1.0 and 1.3. We also determine the evolution of Lyα absorption lines at redshifts less than 1.3 by combining the results for 13 smaller redshift quasars discussed in Paper I of this series with the results for the four moderate redshift quasars analyzed in the present paper. Absorption lines were selected, measured, and identified algorithmically using software tested by Monte Carlo simulations. A total of 291 absorption lines, all with a statistical significance above a specified high threshold level, were selected and measured. A total of 145 lines are identified as extragalactic Lyα absorption lines. Ten of the Lyα absorption lines are found at the same redshifts as metal-line systems. Monte Carlo simulations with pseudo-C IV or O VI doublets were carried out to determine the probability that a pair of absorption lines might accidentally have the appropriate separation to be identified as either a C IV or an O VI absorption doublet. The average number of pseudo-C IV doublets found in the real (observed) spectra varies from 0.05 to 2.4 per spectrum within the Lyα forest and is negligible outside the Lyα forest. For z_abs_ <= 1.3, the density of Lyα lines with equivalent widths greater than 0.24 A is adequately fitted by dN/dz = (dN/dz)_0_(1 + z)^gamma^ with (dN/dz)_0_ = 24.3 +/- 6.6 Lyα lines per unit redshift, and γ = 0.58 +/- 0.50 (1 σ uncertainties). This rate of evolution at low redshifts is less than the evolutionary rate inferred from several different ground-based data samples that pertain to high redshifts, although neither the available HST data nor the ground-based data are sufficiently extensive to establish whether this change occurs abruptly or gradually. The four Lyman-limit systems that are present in the spectra analyzed here all correspond to extensive metal-line systems. This result provides further circumstantial evidence that many Lyman-limit systems (like many metal-line absorption systems) are associated with galaxies. Eight extensive metal-line systems with between five and 15 strong metal lines are identified. An approximate estimate for the frequency of such systems is dN/dz ~ 2.5(1 + z)^0.5^ systems per unit redshift of dN/dz ~ 2.0(1 + z)^1.0^ systems per unit redshift. About half of the extensive metal-line systems are accompanied by clumps of neighboring (in redshift space) Lyα absorption lines, corresponding to velocity dispersions of 600-1400 km s^-1^. In addition, two of the extensive metal systems, found in the spectrum PKS 0122-00 at z = 0.9667 and z = 0.9531, are probably physically associated, since they are separated by only 2000 km s^-1^. We suggest that the metal-line systems with associated clumps of Lyα lines and the linked pair of metal-line systems seen in the spectrum of PKS 0122-00, may correspond to clusters, or possibly superclusters, of galaxies. The observed gaseous structures at redshifts of 0.5-1.0 with velocity dispersions of 6 x 102 to 1.4 x 10- km ski (or velocity spans of 1.2 x 10- to 3 x 10- km ~/1) constitute a constraint on cosmological models of structure formation. The local mean free path (the reciprocal of the number density times radius squared) for the clumps of Ly(alpha) absorptions and metal-line systems is 10^-4^ Mpc^-1^. The clumps of Lyα absorption lines clustered about metal-line systems and the inferred rate of evolution of low and moderate redshift Lyα absorption lines more clearly resemble the properties of galaxies and of metal containing absorption line systems than they do the properties of the high-redshift Lyα forest lines. These results are consistent with two different populations of Lyα absorption lines, with type 1 being closely associated with galaxies and evolving slowly and type 2 being relative unclustered, evolving more rapidly, and dominating the observations at large redshift.