The local Lyα forest.: IV.: Space telescope imaging spectrograph G140M spectra and results on the distribution and baryon content of HI absorbers

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 Lyalpha forest. With STIS, we detect 109 Lyalpha absorbers at significance level (SL) of greater than or equal to 4 sigma over 0.002 < z < 0.069, with a total redshift path length Deltaz = 0.770. Our combined sample consists of 187 Lyalpha absorbers with SL greater than or equal to 4 sigma over Deltaz = 1.157. We evaluate the physical properties of these Lyalpha absorbers and compare them to their high-z counterparts. Using two different models for Lyalpha forest absorbers, we determine that the warm, photoionized intergalactic medium contains 29% +/- 4% of the total baryon inventory at z = 0 (assuming J(0) = 1.3 x 10(-23) ergs cm(-2) s(-1) Hz(-1) sr(-1)). We derive the distribution in column density, N-H I(-1.65 +/- 0.07) for 12.5 less than or equal to log [N-H I(cm(-2)) less than or equal to 14.5, breaking to a flatter slope above log [N-H I] approximate to 14.5. As with the high equivalent width (W > 240 mAngstrom) 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 > 240 mAngstrom 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 Lyalpha clouds is not as great as previously measured, and the break to slower evolution may occur later than previously suggested (z similar to 1.0 rather than 1.6). We find a 7.2 sigma excess in the two-point correlation function (TPCF) of Lyalpha absorbers for velocity separations Deltav less than or equal to 260 km s(-1), which is exclusively due to the higher column density clouds. From our previous result that higher column density Lyalpha 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 similar toroot2.