Metal-line emission from the warm-hot intergalactic medium: II.
  Ultraviolet
  Approximately half the baryons in the local Universe are thought to reside in
the warm-hot intergalactic medium (WHIM). Emission lines from metals in the UV
band are excellent tracers of the cooler fraction of this gas. We present
predictions for the surface brightness of a sample of UV lines that could
potentially be observed by the next generation of UV telescopes at z<1. We use
a subset of simulations from the OWLS project to create emission maps and to
investigate the effect of varying the physical prescriptions for star
formation, supernova and AGN feedback, chemodynamics and radiative cooling.
Most models produce results in agreement within a factor of a few, indicating
that the predictions are robust. Of the lines we consider, C III is the
strongest line, but it typically traces gas colder than 10^5 K. The same is
true for Si IV. The second strongest line, C IV, traces circum-galactic gas
with T~10^5 K. O VI and Ne VIII probe the warmer (T~10^5.5 K and T~10^6 K,
respectively) and more diffuse gas that may be a better tracer of the large
scale structure. N V emission is intermediate between C IV and O VI. The
intensity of all emission lines increases strongly with gas density and
metallicity, and for the bright emission it is tightly correlated with the
temperature for which the line emissivity is highest. In particular, the C III,
C IV, Si IV and O VI emission that is sufficiently bright to be potentially
detectable in the near future (>10^3 photon/s/cm^2/sr), comes from relatively
dense (rho>10^2 rho_mean) and metal rich (Z>0.1 Z_sun) gas. As such, emission
lines are highly biased tracers of the missing baryons and are not an optimal
tool to close the baryon budget. However, they do provide a powerful means to
detect the gas cooling onto or flowing out of galaxies and groups. (Abridged)
