The Hubble Space Telescope quasar absorption line key project. XIV. The evolution of Ly alpha absorption lines in the redshift interval z=0-1.5

We present the results of an analysis of the rate of evolution of the Ly al pha absorption lines in the redshift interval 0.0 to similar to 1.5 based u pon a sample of 987 Ly alpha absorption lines identified in the spectra of 63 QSOs obtained with the Faint Object Spectrograph (FOS) of the Hubble Spa ce Telescope (HST). These spectra were obtained as part of the QSO Absorpti on Line Survey, an HST Key Project during the first four years of observati ons with the telescope. Fits to the evolution of the number of absorbers pe r unit redshift (dN/dz) of the form dN/dz = A x (1 + z)(gamma) continue to yield values of gamma in the range 0.1-0.3, decidedly flatter than results from ground-based data pertaining to the redshift range z > 1.7. These resu lts are consistent with our previous results based on a much smaller sample of lines, but the uncertainties in the fit have been greatly reduced. The combination of the HST and ground-based data suggest a marked transition in the rate of evolution of the Ly alpha lines at a redshift of about 1.7. Th e 19 Ly alpha lines from an additional higher redshift QSO from our sample for which tentative line identifications are available (UM 18; z(em) = 1.89 ) support the suggestion of a rapid increase at around this redshift. We de rive the cumulative distribution of the full sample of Ly alpha lines and s how that the distribution in redshift can indeed be well represented by a p ower law of the form (1 + z)(gamma). For this same sample, the distribution of equivalent widths of the Ly alpha absorbers above a rest equivalent wid th of 0.1 Angstrom is fit quite well by an exponential. Comparing samples o f Ly alpha lines, one set of which has redshifts the same as, or very near to, the redshifts of ions from heavy elements and another set in which no i ons from heavy elements have been identified, we find that the Ly alpha sys tems with heavy element detections have a much steeper slope than the high rest equivalent width portion of the Lyman-only sample. We argue that this result is not likely to be due to either line misidentification or incomple te spectral coverage. Considering the insensitivity of the equivalent width to large changes in the column density for saturated lines, we suggest tha t this result is probably attributable to rapid evolution of the very highe st column density systems, rather than real differences in metallicity. We find evidence that the rate of evolution increases with increasing equivale nt width. We compare our results for the variation of line density with red shift to recent numerical simulations of Ly alpha absorbers, in particular, to those of Riediger, Petitjean, & Mucket, which extend to zero redshift. We find fairly good agreement between these simulations and our results, th ough the rapid evolution we find in the Ly alpha systems containing heavy e lement ions is not predicted in their models. We speculate that these heavy element-containing Ly alpha systems involve those clouds closely associate d with galaxies, whose column densities are too high and whose sizes are to o small to be included in the Riediger et al. simulations. Our results for Ly alpha lines at the high end of our equivalent width distribution are com patible with the recent analysis of the absorber-galaxy correlation by Chen et al. For the weaker lines, however, our results suggest that whatever as sociation exists between absorbers and galaxies is different from that for the stronger lines. We conclude with some suggestions for further observati ons.