The Local Lyα Forest. I. Observations with the GHRS/G160M on the Hubble Space Telescope

We present the target selection, observations, and data reduction and analysis process for a program aimed at discovering numerous, weak (equivalent width <=100 mÅ) Lyα absorption lines in the local universe (0.003<=z<=0.069). The purpose of this program is to study the physical conditions of the local intergalactic medium, including absorber distributions in Doppler width and H I column density, redshift evolution of absorber number density, line-of-sight two-point correlation function, and the baryonic content and metallicity. By making use of large-angle, nearby galaxy redshift surveys, we will investigate the relationship (if any) between these Lyα absorbers and galaxies, superclusters, and voids. In Paper I we present high-resolution (~19 km s^-1) spectroscopic observations of 15 very bright (V<=14.5) active galactic nuclei (AGNs) targets made with the Goddard High-Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST). We find 81 definite (>=4 σ) and 30 possible (3-4 σ) Lyα absorption lines in these spectra, which probe a total path length of 116,000 km s^-1 (Δz~0.4) at very low redshift (z<=0.069) and column density (12.5<=logN_HI<=14.5). We found numerous metal lines arising in the Milky Way halo, including absorption from high-velocity clouds in 10 of 15 sight lines and numerous absorptions intrinsic to the target AGN. We present reduced spectra, absorption-line lists, ``pie diagrams,'' showing the known galaxy distributions in the direction of each target, and nearest galaxy tables for each absorber. In Papers II and III, we use the data presented here to determine the basic physical characteristics of the low-z Lyα forest and to investigate the relationship of the absorbers to the local galaxy distribution. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.