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 =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=1.3×10-23 ergs cm-2 s-1 Hz-1 sr-1). We derive the distribution in column density, N-1.65+/-0.07HI for 12.5 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