The local Ly alpha 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 less than or equal to 100 m Angstrom) Ly alpha absorption lines in the local universe (0.003 less than or equal to z less than or equal to 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 Ly alpha absorbers and galaxies, superclusters, and voids. In Paper I we present high-resolution (similar to 19 km s(-1)) spectroscopic observations of 15 very bright (V less than or equal to 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 (greater than or equal to4 sigma) and 30 possible (3-4 sigma) Ly alpha absorption lines in these spectra, which probe a total path length of 116,000 km s(-1) (Deltaz similar to 0.4) at very low redshift (z less than or equal to 0.069) and column density (12.5 less than or equal to log N-HI less than or equal to 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 TI and III, we use the data presented here to determine the basic physical characteristics of the low-z Ly alpha forest and to investigate the relationship of the absorbers to the local galaxy distribution.