A uniform analysis of the Lyα forest at z = 0-5. IV. The clustering and evolution of clouds at z ≤ 1.7

We present results on the evolution and clustering of Lyα lines at low redshift as part of our series "A Uniform Analysis of the Lyα Forest at z = 0-5." The sample analyzed in this paper contains 1298 Lyα absorption lines from 165 quasar spectra mined from the archives of the Faint Object Spectrograph on the Hubble Space Telescope (HST). Our sample extends to z ≈ 1.7, slightly higher than the sample analyzed by the HST Quasar Absorption Line Key Project. We confirm the result from the Key Project that the number density evolution of Lyα lines at low redshifts can be described by a power law that is significantly flatter than that found at high z. We find that the evolution is somewhat steeper than obtained previously. Specifically, we find γ = 0.54 ± 0.21 for lines with equivalent widths greater than 0.24 Å and γ = 0.60 ± 0.14 using a variable equivalent width threshold. We find that the difference between our and Key Project results is likely attributable to different redshift coverage of the two samples. The results concerning the number density evolution are not significantly affected if one includes Lyα lines which are members of metal systems. Object-to-object fluctuations in the number of lines detected are small, indicating a high degree of uniformity in the intergalactic medium on large scales. We find marginal evidence that weak and strong lines undergo different evolution. We find weak clustering for Lyα lines at velocity separations Δ V ≤ 500 km s^-1, weaker than the level predicted from an earlier analysis by Ulmer of a small subsample of the Key Project data. We see no correlations for metal system-Lyα forest or extensive metal system-Lyα forest combinations.