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 /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 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 d ℓ/ 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 ⩾ 10 6 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.