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.
