THE BARYON CENSUS IN A MULTIPHASE INTERGALACTIC MEDIUM: 30% OF THE BARYONS MAY STILL BE MISSING

Although galaxies, groups, and clusters contain similar to 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) Ly alpha forest and warm-hot IGM (WHIM) at 10(5-6) K traced by O VI lambda 1032, 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(circle dot)) and ionization fractions (f(H I), f(O VI)). Statistically, the O VI correction product correlates with column density, (Z/Z(circle dot)) f(O VI) approximate to (0.015)(N-O VI/10(14) 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 Ly alpha 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 Ly alpha forest (28% +/- 11%) and WHIM traced by O VI and broad-Ly alpha 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 similar to 15% reside in hotter WHIM (T >= 10(6) K). Additional baryons could be detected in weaker Ly alpha 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.