Strong z~0.5 O VI Absorption Toward PKS 0405-123: Implications for
  Ionization and Metallicity of the Cosmic Web
  We present observations of the O VI system at z_abs = 0.495096 toward PKS
0405-123 (z_em = 0.5726) obtained with FUSE and STIS. In addition to strong O
VI, with log N(O VI) = 14.47+/-0.02, and moderate H I this absorber shows C
III, N IV, O IV, and O V, with upper limits for another seven ions. The large
number of available ions allows us to test ionization models usually adopted
with far less contraints. We find the ionic column densities cannot be matched
by collisional ionization models, in or out of equilibrium. Photoionization
models can match the observed column densities, including O VI. If one assumes
photoionization by a UV background dominated by QSOs, the metallicity of the
gas is [O/H] = -0.15, while a model for the UV background with contributions
from ionizing photons escaping from galaxies gives [O/H] = -0.62. Both give
[N/O] ~ -0.6 and [C/H] ~ -0.2 to -0.1. The choice of ionizing spectrum is
poorly constrained. Multiphase models with a contribution from both
photoionized gas (at T~10^4 K) and collisionally ionized gas (at T~(1-3)x10^5
K) can also match the observations giving very similar metallicities. The O VI
in this system is not necessarily a reliable tracer of WHIM matter. We do not
detect Ne VIII or Mg X absorption. The limit on Ne VIII/O VI < 0.21 (3 sigma),
the lowest yet observed. Thus this absorber shows no firm evidence of the
"warm-hot intergalactic medium" at T~(0.5-3)x10^6 K thought to contain a
significant fraction of the baryons at low redshift. We present limits on the
total column of warm-hot gas in this absorber as a function of temperature.
This system would be unlikely to provide detectable X-ray absorption in the
ions O VII or O VIII even if it resided in front of the brighter X-ray sources
in the sky.
