2012ApJ...759...23S

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/cm2)^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/cm2, N_ O VI_ ≥ 10^12.5/cm2, N_O VII_ ≥ 10^14.5/cm2, using high signal-to-noise data from high-resolution UV and X-ray spectrographs.