  We present results from an analysis of FUSE and HST/STIS spectroscopy of the
z_em=0.57 quasar PKS0405-123. We focus on the intervening metal-line systems
identified along the sightline and investigate their ionization mechanism,
ionization state, and chemical abundances. We identify six OVI absorbers to a
3sigma 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 systems
we report positive detections of CIII suggesting the gas is predominantly
photoionized, has multiple ionization phases, or is in a non-equilibrium state.
This contrasts with the general description of the warm-hot intergalactic
medium (WHIM) as described by numerical simulations where the gas is
predominantly in collisional ionization equilibrium. We have also searched the
sightline for the NeVIII doublet (a better probe of WHIM gas with T>10^6K) over
the redshift range 0.2<z<z_em and report no positive detections to an EW limit
of 80mA giving dN/dz<40 at 95% c.l. The photoionized metal-line systems exhibit
a correlation between the ionization parameter (U) and HI column density for
N(HI) = 10^(14-16) cm^-2. Both the slope and normalization of this correlation
match the prediction inferred from the results of Dave and Tripp for the low z
Lya forest. In turn, the data show a tentative, unexpected result: five out of
the six photoionized metal-line systems show a total hydrogen column density
within a factor of 2 of 10^18.7 cm^-2. Finally, the median metallicity [M/H] of
twelve z~0.3 absorbers with N(HI)>10^14 cm^-2 is [M/H] ~> -1.5 with large
scatter. This significantly exceeds the median metallicity of CIV and OVI
systems at z~3 and indicates enrichment of the intergalactic medium over the
past 10Gyr. [abriged]
