Where Are the Baryons? II. Feedback Effects

Numerical simulations of the intergalactic medium have shown that at the present epoch, a significant fraction (40%-50%) of the baryonic component should be found in the (T ~ 10⁶ K) warm-hot intergalactic medium (WHIM)—with several recent observational lines of evidence indicating the validity of the prediction. We here recompute the evolution of the WHIM with the following major improvements: (1) galactic superwind feedback processes from galaxy and star formation are explicitly included; (2) major metal species (O V to O IX) are computed explicitly in a nonequilibrium way; and (3) mass and spatial dynamic ranges are larger by factors of 8 and 2, respectively, than in our previous simulations. Here are the major findings: (1) Galactic superwinds have dramatic effects, increasing the WHIM mass fraction by about 20%, primarily through heating of warm gas near galaxies with density 10^1.5-10⁴ times the mean density. (2) The fraction of baryons in the WHIM is increased modestly from the earlier work but is still ~40%-50%. (3) The gas density of the WHIM is broadly peaked at a density 10-20 times the mean density, ranging from underdense regions to regions that are overdense by 10³-10⁴. (4) The median metallicity of the WHIM is 0.18 Z_☉ for oxygen, with 50% and 90% intervals being (0.040, 0.38) and (0.0017, 0.83).