We present results on the evolution and clustering of Ly- \alpha lines at low redshift as part of our series “ A uniform analysis of the Ly- \alpha forest at z = 0 - 5 . ” The sample analyzed in this paper contains 1298 Ly- \alpha 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 \approx 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 Ly- \alpha 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 \gamma = 0.54 \pm 0.21 for lines with equivalent widths greater than 0.24 Å and \gamma = 0.60 \pm 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 Ly- \alpha 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 Ly- \alpha lines at velocity separations \Delta V \leq 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–Ly- \alpha forest or extensive metal system–Ly- \alpha forest combinations .