Probing the Intergalactic Medium–Galaxy Connection toward PKS 0405−123. I. Ultraviolet Spectroscopy and Metal‐Line Systems

We present results from an analysis of FUSE and HST/STIS spectroscopy of thez_em=0.57 quasar PKS0405-123. We focus on the intervening metal-line systemsidentified along the sightline and investigate their ionization mechanism,ionization state, and chemical abundances. We identify six OVI absorbers to a3sigma equivalent width (EW) limit of 60mA. This implies an incidence dN/dz =16^{+9}_{-6} consistent with previous OVI studies. In half of the OVI systemswe report positive detections of CIII suggesting the gas is predominantlyphotoionized, has multiple ionization phases, or is in a non-equilibrium state.This contrasts with the general description of the warm-hot intergalacticmedium (WHIM) as described by numerical simulations where the gas ispredominantly in collisional ionization equilibrium. We have also searched thesightline for the NeVIII doublet (a better probe of WHIM gas with T>10^6K) overthe redshift range 0.210^14 cm^-2 is [M/H] ~> -1.5 with largescatter. This significantly exceeds the median metallicity of CIV and OVIsystems at z~3 and indicates enrichment of the intergalactic medium over thepast 10Gyr. [abriged]Comment: 21 pages, 21 figures (mostly color). Accepted to ApJ, August 12, 200