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 = 10⁵ to 10⁷ K. Current tracers of WHIM at 10⁵ to 10⁶ 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 (fOVI) using cosmological simulations of heating, cooling, and metal transport in a density-temperature structured medium. Statistically, their product correlates with column density, (Z/Zsun) (fOVI) = (0. 015) (NOVI/10^14 cm^-2) ⁰. 70. The NOVI-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 NHI > 10^12. 0 cm^-2, NOVI > 10^12. 5 cm^-2, and NOVII > 10^14. 5 cm^-2, with moderate-resolution UV and X-ray spectrographs.