Book Description

We present results on the evolution of Ly[alpha] absorption at low redshift, and the first systematic search for Si II absorption systems in the low redshift IGM. Our sample consists of 832 Ly[alpha] absorbers from 328 spectra of 204 QSOs taken from the Hubble Space Telescope archive. We develop a new, reproducible method of quasar continuum fitting, designed to quickly identify absorption lines and measure the relative line strength (a proxy for equivalent width). Our method, which fails to identify the weakest lines, does manage to detect the strong features in a given spectrum and provides enough information to identify metal absorption line systems. We confirm the results of previous studies of Ly[alpha] evolution at low redshift and find the number density of absorbers can be described by a power law in (l+z) that is much flatter than that found for Lya evolution at high redshift. Specifically, we measure a power law index of [gamma] = 0.57 ± 0.16 for lines with a rest relative line strength greater than 0.10 [Angstroms]. We also identify the presence of 14 Si II systems at z [approx] 1. The number of Si ii 1193 and 1260 [Angstroms] systems per unit redshift path length at a mean redshift of z = 0.9 is N(z)= 1.6 ± 0.6. This density is similar to that found for C Ii, Mg ii, and O vi absorbers.