2004ApJS..152...29P

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<z<0.069, with a total redshift path length Δz=0.770. Our combined sample consists of 187 Lyα absorbers with SL≥4σ over Δz=1.157. We evaluate the physical properties of these Lyα absorbers and compare them to their high-z counterparts. Using two different models for Lyα forest absorbers, we determine that the warm, photoionized intergalactic medium contains 29%±4% of the total baryon inventory at z=0 (assuming J₀=1.3x10^–23 ergs/cm2/s/Hz/sr). We derive the distribution in column density, N^{–1.65±0.07}_HI_for 12.5≤logN_HI_/(cm2)≤14.5, breaking to a flatter slope above logN_HI_~14.5. As with the high equivalent width (W>240mÅ) absorbers, the number density of low-W absorbers at z=0 is well above the extrapolation of dN/dz from z>2. However, log(dN/dz)_z=0=1.40±0.08 for W>240mÅ 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-W 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≤260km/s, 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 ~sqrt(2).