The Local Lyα Forest. IV. Space Telescope Imaging Spectrograph G140M Spectra and Results on the Distribution and Baryon Content of H i Absorbers

Abstract

We present Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) G140M spectra of 15 extragalactic targets, which we combine with GHRS/G160M data to examine the statistical properties of the low-z Lyα forest. With STIS, we detect 109 Lyα absorbers at significance level (SL) of ≥4 σ over 0.002 240 mA) absorbers, the number density of low- absorbers at z = 0 is well above the extrapolation of d/dz from z > 2. However, log = 1.40 ± 0.08 for > 240 mA is 25% below the value obtained by the HST QSO Key Project, a difference that may arise from line blending. The slowing of the number density evolution of high- Lyα clouds is not as great as previously measured, and the break to slower evolution may occur later than previously suggested (z ~ 1.0 rather than 1.6). We find a 7.2 σ excess in the two-point correlation function (TPCF) of Lyα absorbers for velocity separations Δv ≤ 260 km s-1, which is exclusively due to the higher column density clouds. From our previous result that higher column density Lyα clouds cluster more strongly with galaxies, this TPCF suggests a physical difference between the higher and lower column density clouds in our sample. The systematic error produced by cosmic variance on these results increases the total errors on derived quantities by ~.