THE HUBBLE-SPACE-TELESCOPE QUASAR ABSORPTION-LINE KEY PROJECT .7. ABSORPTION SYSTEMS AT Z(ABS)LESS-THAN-OR-EQUAL-TO-1.3

We present evidence that clumps of Ly alpha lines are physically assoc iated with about half of the extensive metal-line systems (absorption systems with four or more observed metal-line species) found in this p aper, demonstrate that all four Lyman-limit systems discussed here cor respond to extensive metal-line absorption systems, and present an ext raordinary pair of extensive metal-line absorption systems within 2000 km s(-1) of each other at z = 0.95 that are probably an early manifes tation of large-scale structure. These results are obtained using ultr aviolet spectra, taken with the higher resolution gratings of the Fain t Object Spectrograph of the Hubble Space Telescope (HST) for four qua sars with emission-line redshifts between 1.0 and 1.3. We also determi ne the evolution of Ly alpha 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 redsh ift quasars analyzed in the present paper. Absorption lines were selec ted, 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 extragal actic Ly alpha absorption lines. Ten of the Ly alpha absorption lines are found at the same redshifts as metal-line systems. Monte Carlo sim ulations with pseudo-C IV or O VI doublets were carried out to determi ne the probability that a pair of absorption lines might accidentally have the appropriate separation to be identified as either a C IV or a n 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 spect rum within the Ly alpha forest and is negligible outside the Ly alpha forest. For z(abs) less than or equal to 1.3, the density of Ly alpha lines with equivalent widths greater than 0.24 Angstrom is adequately fitted by dN/dz = (dN/dz)(0)(1 + z)(y) with (dN/dz)(0) = 24.3 +/- 6.6 Ly alpha lines per unit redshift, and y = 0.58 +/- 0.50 (1 sigma uncer tainties). This rate of evolution at low redshifts is less than the ev olutionary rate inferred from several different ground-based data samp les that pertain to high redshifts, although neither the available HST data nor the ground-based data are sufficiently extensive to establis h whether this change occurs abruptly or gradually. The four Lyman-lim it systems that are present in the spectra analyzed here all correspon d to extensive metal-line systems. This result provides further circum stantial evidence that many Lyman-limit systems (like many metal-line absorption systems) are associated with galaxies. Eight extensive meta l-line systems with between five and 15 strong metal lines are identif ied. An approximate estimate for the frequency of such systems is dN/d z similar or equal to 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 exte nsive metal-line systems are accompanied by clumps of neighboring (in redshift space) Ly alpha absorption lines, corresponding to velocity d ispersions of 600-1400 km s(-1). In addition, two of the extensive met al 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 associ ated clumps of Ly alpha lines and the linked pair of metal-line system s seen in the spectrum of PKS 0122-00, may correspond to clusters, or possibly superclusters, of galaxies. The observed gaseous structures a t redshifts of 0.5-1.0 with velocity dispersions of 6 x 10(2) to 1.4 x 10(3) km s(-1) (or velocity spans of 1.2 x 10(3) to 3 x 10(3) km s(-1 )) constitute a constraint on cosmological models of structure formati on. The local mean free path (the reciprocal of the number density tim es radius squared) for the clumps of Ly alpha absorptions and metal-li ne systems is 10(-4) Mpc(-1). The clumps of Ly alpha absorption lines clustered about metal-line systems and the inferred rate of evolution of low and moderate redshift Ly alpha absorption lines more nearly res emble the properties of galaxies and of metal-containing absorption-li ne systems than they do the properties of the high-redshift Ly alpha f orest lines. These results are consistent with two different populatio ns of Ly alpha absorption lines, with type 1 being closely associated with galaxies and evolving relatively slowly and type 2 being relative unclustered, evolving more rapidly, and dominating the observations a t large redshift.