Characterizing the Warm-Hot Intergalactic Medium at High Redshift: A High-Resolution Survey for O VI at z = 2.5

We have conducted a survey for warm-hot gas, traced by O VI absorption in the spectra of five high-redshift quasars (2.2 < z < 2.8) observed with Keck I/HIRES. We identify 18 O VI systems, 12 of which comprise the principal sample for this work. Of the remaining six systems, two are interpreted as ejecta from the QSO central engine, and four have ionization conditions affected by proximity to the background QSO. Of the 12 intergalactic O VI absorbers, 11 are associated with complex systems showing strong Lyα (N > 1015.2 cm-2), C IV, and often other lower ionization species. We do not detect any lines that resemble photoionized, enriched gas associated with the lowest density regions of the Lyα forest (13.5 < log N < 14.5). Not all of the systems lend themselves to a straightforward determination of ionization conditions, but in general we find that they most closely resemble hot, collisionally ionized gas found near regions of significant overdensity. The extent and gas density of the intergalactic O VI absorbing regions are constrained to be L ≤ 200 kpc and ρ/ ≥ 2.5. This was calculated by comparing the maximum observed O VI line width with the broadening expected for clouds of different sizes due to the Hubble flow. For the median observed value of the Doppler parameter b = 16 km s-1, the inferred cloud sizes and densities are L ~ 60 kpc and ρ/ ~ 10-30. The clouds have at least two distinct gas phases. One gives rise to absorption in photoionized C IV and Si IV and has temperatures in the range T = 20,000-40,000 K and overdensities of ρ/ ≥ 100. The second phase is traced only in O VI absorption. Its temperature is difficult to constrain because of uncertainties in the nonthermal contribution to line broadening. However, the distribution of upper limits on the O VI, C IV, and Si IV temperatures indicates that the O VI thermal structure differs from that of the other ions and favors higher temperatures where collisional ionization would be significant. The O VI systems are strongly clustered on velocity scales of Δv = 100-300 km s-1 and show weaker clustering out to Δv = 750 km s-1. The power-law slope of the two-point correlation function is similar to that seen from local galaxy and cluster surveys, with a comoving correlation length of ~11 h Mpc. The average oxygen abundance of the O VI systems is constrained to be [O/H] ≥ -1.5 at z ~ 2.5, about 10 times higher than the level observed in the general intergalactic medium. Two production mechanisms for the hot gas are considered: shock heating of pre-enriched gas falling onto existing structure, and expulsion of material by supernova-driven galactic winds. Comparison between the observed numbers of O VI systems and expectations from simulations indicates that infall models tend to overproduce O VI lines by a factor of ~10, although this discrepancy might be resolved in larger, higher resolution calculations. Known galaxy populations such as the Lyman break objects are capable of producing the amount of O VI absorption seen in the survey, provided that they drive winds to distances of R ~ 50 kpc.