The Hubble Space Telescope Quasar Absorption Line Key Project. XIV. The Evolution of Lyα Absorption Lines in the Redshift Interval z = 0-1.5^*

We present the results of an analysis of the rate of evolution of the Lyα absorption lines in the redshift interval 0.0 to ~1.5 based upon a sample of 987 Lyα absorption lines identified in the spectra of 63 QSOs obtained with the Faint Object Spectrograph (FOS) of the Hubble Space Telescope (HST). These spectra were obtained as part of the QSO Absorption Line Survey, an HST Key Project during the first four years of observations with the telescope. Fits to the evolution of the number of absorbers per unit redshift (dN/dz) of the form dN/dz = A × (1 + z)^γ continue to yield values of γ in the range 0.1-0.3, decidedly flatter than results from ground-based data pertaining to the redshift range z > 1.7. These results 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 Lyα lines at a redshift of about 1.7. The 19 Lyα 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 derive the cumulative distribution of the full sample of Lyα lines and show that the distribution in redshift can indeed be well represented by a power law of the form (1 + z)^γ. For this same sample, the distribution of equivalent widths of the Lyα absorbers above a rest equivalent width of 0.1 Å is fit quite well by an exponential. Comparing samples of Lyα 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 ions from heavy elements have been identified, we find that the Lyα systems 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 incomplete spectral coverage. Considering the insensitivity of the equivalent width to large changes in the column density for saturated lines, we suggest that this result is probably attributable to rapid evolution of the very highest column density systems, rather than real differences in metallicity. We find evidence that the rate of evolution increases with increasing equivalent width. We compare our results for the variation of line density with redshift to recent numerical simulations of Lyα 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, though the rapid evolution we find in the Lyα systems containing heavy element ions is not predicted in their models. We speculate that these heavy element-containing Lyα systems involve those clouds closely associated with galaxies, whose column densities are too high and whose sizes are too small to be included in the Riediger et al. simulations. Our results for Lyα lines at the high end of our equivalent width distribution are compatible with the recent analysis of the absorber-galaxy correlation by Chen et al. For the weaker lines, however, our results suggest that whatever association exists between absorbers and galaxies is different from that for the stronger lines. We conclude with some suggestions for further observations.