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Coincidences between OVI and OVII Lines: Insights from High Resolution Simulations of the Warm-Hot Intergalactic Medium

With high resolution (0.46kpc/h), adaptive mesh-refinement Eulerian cosmological hydrodynamic simulations we compute properties of O VI and O VII absorbers from the warm-hot intergalactic medium (WHIM). Our new simulations are in broad agreement with previous simulations, with ~40% of the intergalactic medium being in the WHIM at z=0. It is found (1) The amount of gas in the WHIM at temperature below and above 10^6K is about equal within uncertainties. (1) Our simulations are in excellent agreement with observed properties of O VI absorbers, with respect to the line incidence rate and Doppler width-column density relation. (2) Velocity structures within absorbing regions are a significant, and for large Doppler width clouds, a dominant contributor to the Doppler widths of both O VI and O VII absorbers. A non-negligible fraction (in number and mass) of O VI and O VII clouds can arise from gas of temperature lower than 10^5, until the Doppler width is well in excess of 100km/s. (3) Strong O VI absorbers are predominantly collisionally ionized. About (61%, 57%, 39%) of O VI absorbers in the column density ranges of log N(OVI) cm^2=(12.5-13,13-14,>14) have temperature lower than 10^5K. (4) Quantitative prediction is made for the presence of broad and shallow O VI lines, which current observations may have largely missed. Upcoming observations by COS may be able to provide a test. (5) The reported 3 sigma upper limit on the mean column density of coincidental O VII lines at the location of detected O VI lines by Yao et al is above the predicted value by a factor of 2.5-4. (6) The claimed observational detection of O VII lines by Nicastro et al, if true, is 2 sigma above what our simulations predict.