STAR FORMATION FEEDBACK AND METAL-ENRICHMENT HISTORY OF THE INTERGALACTIC MEDIUM

Using the state-of-the-art cosmological hydrodynamic simulations of the standard cold dark matter model with star formation feedback strength normalized to match the observed star formation history of the universe at z = 0-6, we compute the metal-enrichment history of the intergalactic medium (IGM). Overall we show that galactic superwind (GSW) feedback from star formation can transport metals to the IGM and that the properties of simulated metal absorbers match current observations. The distance of influence of GSW from galaxies is typically limited to about <= 0.5 Mpc and within regions of overdensity delta >= 10. Most C IV and O VI absorbers are located within shocked regions of elevated temperature (T >= 2 x 10(4) K), overdensity (delta >= 10), and metallicity ([Z/Z(circle dot)] = [-2.5,-0.5]), enclosed by double shocks propagating outward. O VI absorbers have typically higher metallicity, lower density, and higher temperature than C IV absorbers. For O VI absorbers, collisional ionization dominates over the entire redshift range z = 0-6, whereas for C IV absorbers the transition occurs at moderate redshift z similar to 3 from collisionally dominated to photoionization dominated. We find that the observed column density distributions for C IV and O VI in the range log N cm(2) = 12-15 are reasonably reproduced by the simulations. The evolution of mass densities contained in C IV and O VI lines, Omega(CIV) and Omega(OVI), is also in good agreement with observations, which shows a near constancy at low redshifts and an exponential drop beyond redshift z = 3-4. For both C IV and O VI, most absorbers are transient and the amount of metals probed by C IV and O VI lines of column log N cm(2) = 12-15 is only similar to 2% of total metal density at any epoch. While gravitational shocks from large-scale structure formation dominate the energy budget (80%-90%) for turning about 50% of the IGM to the warm-hot intergalactic medium (WHIM) by z = 0, GSW feedback shocks are energetically dominant over gravitational shocks at z >= 1-2. Most of the so-called missing metals at z = 2-3 are hidden in a warm-hot (T = 10(4.5)-10(7) K) gaseous phase, heated up by GSW feedback shocks. Their mass distribution is broadly peaked at delta = 1-10 in the IGM, outside virialized halos. Approximately 37%, 46%, 10%, and 7% of the total metals at z = 0 are in stars, WHIM, X-ray gas, and cold gas, respectively; the distributions stand at 23%, 57%, 2%, and 18% and 14%, 51%, 4%, and 31% at z = 2 and z = 4, respectively.