The Baryon Census in a Multiphase Intergalactic Medium: 30% of the
  Baryons May Still Be Missing
  For low-redshift cosmology and galaxy formation rates, it is important to
account for all the baryons synthesized in the Big Bang. Although galaxies and
clusters contain 10% of the baryons, many more reside in the photoionized
Lyman-alpha forest and shocked-heated warm-hot intergalactic medium (WHIM) at T
= 10^5 to 10^7 K. Current tracers of WHIM at 10^5 to 10^6 K include the O VI
1032, 1038 absorption lines, together with broad Lyman-alpha absorbers (BLAs)
and EUV/X-ray absorption lines from Ne VIII, O VII, and O VIII. We improve the
O VI baryon surveys with corrections for oxygen metallicity (Z/Zsun) and O VI
ionization fraction (f_OVI) using cosmological simulations of heating, cooling,
and metal transport in a density-temperature structured medium. Statistically,
their product correlates with column density, (Z/Zsun)(f_OVI) =
(0.015)(N_OVI/10^{14} cm^-2)^0.70. The N_OVI-weighted mean is 0.01, which
doubles previous estimates of WHIM baryon content. We also reanalyze H I data
from the Hubble Space Telescope, applying redshift corrections for absorber
density, photoionizing background, and proper length, dl/dz. We find
substantial baryon fractions in the photoionized Lya forest (28 +/- 11%), O
VI/BLA-traced WHIM (25 +/- 8%), and collapsed phase (18 +/- 4%) in galaxies,
groups, clusters, and circumgalactic gas. The baryon shortfall is 29 +/- 13%,
which may be detected in X-ray absorbers from hotter WHIM or in weaker Lya and
O VI absorbers. Further progress will require higher-precision baryon surveys
of weak absorbers at column densities N_HI > 10^{12.0} cm^-2, N_OVI > 10^{12.5}
cm^-2, and N_OVII > 10^{14.5} cm^-2, with moderate-resolution UV and X-ray
spectrographs.
