The Baryon Census in a Multiphase Intergalactic Medium: 30% of the Baryons May Still be Missing

Although galaxies, groups, and clusters contain ~10% of the baryons, many more reside in the photoionized and shocked-heated intergalactic medium (IGM) and in the circumgalactic medium (CGM). We update the baryon census in the (H I) Lyα forest and warm-hot IGM (WHIM) at 10^5-6 K traced by O VI λ1032, 1038 absorption. From Enzo cosmological simulations of heating, cooling, and metal transport, we improve the H I and O VI baryon surveys using spatially averaged corrections for metallicity (Z/Z _⊙) and ionization fractions (f _{H I} , f _{O VI} ). Statistically, the O VI correction product correlates with column density, (Z/Z _⊙)f _{O VI} ≈ (0.015)(N _{O VI} /10¹⁴ cm^-2)^0.70, with an N _{O VI} -weighted mean of 0.01, which doubles previous estimates of WHIM baryon content. We also update the Lyα forest contribution to baryon density out to z = 0.4, correcting for the (1 + z)³ increase in absorber density, the (1 + z)^4.4 rise in photoionizing background, and cosmological proper length dl/dz. We find substantial baryon fractions in the photoionized Lyα forest (28% ± 11%) and WHIM traced by O VI and broad-Lyα absorbers (25% ± 8%). The collapsed phase (galaxies, groups, clusters, CGM) contains 18% ± 4%, leaving an apparent baryon shortfall of 29% ± 13%. Our simulations suggest that ~15% reside in hotter WHIM (T >= 10⁶ K). Additional baryons could be detected in weaker Lyα and O VI absorbers. Further progress requires higher-precision baryon surveys of weak absorbers, down to minimum column densities N _{H I} >= 10^12.0 cm^-2, N _{O VI} >= 10^12.5 cm^-2, N _{O VII} >= 10^14.5 cm^-2, using high signal-to-noise data from high-resolution UV and X-ray spectrographs.