THE ULTRAVIOLET-ABSORPTION SPECTRUM OF THE QUASAR PKS 0405-12 AND THE LOCAL DENSITY OF LYMAN-ALPHA ABSORPTION SYSTEMS

A sample of 32 absorption lines has been identified in the ultraviolet spectrum of the z = 0.57 quasar PKS 0405-12. The observations were obtained using three R = 1300 gratings of the Faint Object Spectrograph of the Hubble Space Telescope. The data cover the wavelength range 1190-3260 angstrom. The sample was constructed by requiring that all included lines have a measured equivalent width greater than the wavelength-dependent 3 sigma detection limit for unresolved lines. Sixteen of the absorption features are due to Galactic interstellar lines. There are 10 extragalactic Lyalpha absorption lines in the complete sample, all with observed equivalent widths greater than or equal to 0.40 angstrom three of the Lyalpha lines have Lybeta counterparts. The number of Lyalpha lines observed in the spectrum of PKS 0405-12 is within 1 sigma of the number predicted on the basis of previous HST observations of 3C 273 and of H1821+643. Combining the HST observations of 3C 273, H 1821+643, and PKS 0405-12, we estimate the local number density of Lyalpha systems with rest equivalent widths larger than 0.32 angstrom to be dN/dz(W(rest) greater-than-or-equal-to 0.32 angstrom) almost-equal-to 15+4 Lyalpha lines per unit redshift. Ground-based images reveal a rich field of galaxies in the direction of PKS 0405-12, including many galaxies with the brightnesses and sizes expected if they belong to a cluster associated with the quasar. The quasar spectrum does not show any evidence for absorption at the redshift of the emission lines, indicating a covering factor of less than unity for the halos of galaxies in the cluster around PKS 0405-12.