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

We present the target selection, observations, and data reduction andanalysis process for a program aimed at discovering numerous, weak (equivalentwidth < 100 mA) Lya absorption lines in the local Universe (0.003 < z < 0.069).The purpose of this program is to study the physical conditions of the localintergalactic medium, including absorber distributions in Doppler width and H Icolumn density, absorber number density and evolution with redshift,line-of-sight two-point correlation function, and the baryonic content andmetallicity. By making use of large-angle, nearby galaxy redshift surveys, wewill investigate the relationship (if any) between these Lya absorbers andgalaxies, superclusters and voids. In Paper I, we present high resolution (~19km/s) spectroscopic observations of 15 very bright (V < 14.5) AGN targets madewith the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble SpaceTelescope (HST). We find 81 definite (> 4 sigma) and 30 possible (3-4 sigma)Lya absorption lines in these spectra, which probe a total pathlength of116,000 km/s (delta z ~ 0.4) at very low redshift (z < 0.069) and columndensity (12.5< Log[Nh] < 14.5). We found numerous metal lines arising in theMilky Way halo, including absorption from 14 distinct high velocity clouds andnumerous absorptions intrinsic to the target AGN. Here, we describe the detailsof the target selection, HST observations, and spectral reduction and analysis. We present reduced spectra and absorption line lists and ``pie diagrams''showing the known galaxy distributions in the direction of each target. InPapers II and III, we use the data presented here to determine the basicphysical characteristics of the low-z Lya forest and to investigate therelationship of the absorbers to the local galaxy distribution.Comment: 98 pages, 10 tables, 35 figures, To Appear in ApJS (Sep 2000) The continuum fits near Ly-alpha emission features are greatly improved, resulting in changes to the line lists and table