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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 0.2<z<z_em and report no positive detections to an EW limitof 80mA giving dN/dz<40 at 95% c.l. The photoionized metal-line systems exhibita correlation between the ionization parameter (U) and HI column density forN(HI) = 10^(14-16) cm^-2. Both the slope and normalization of this correlationmatch the prediction inferred from the results of Dave and Tripp for the low zLya forest. In turn, the data show a tentative, unexpected result: five out ofthe six photoionized metal-line systems show a total hydrogen column densitywithin a factor of 2 of 10^18.7 cm^-2. Finally, the median metallicity [M/H] oftwelve z~0.3 absorbers with N(HI)>10^14 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

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