Metal-line emission from the warm-hot intergalactic medium - II. Ultraviolet

Abstract

Approximately half the baryons in the local Universe are thought to reside in the warm-hot intergalactic medium (WHIM), i.e. diffuse gas with temperatures in the range 105 < T < 107 K. Emission lines from metals in the UV band are excellent tracers of the cooler fraction of this gas, with T≲ 106 K. 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 OverWhelmingly Large Simulations project to create emission maps and to investigate the effect of varying the physical prescriptions for star formation, supernova and active galactic nuclei (AGN) feedback, chemodynamics and radiative cooling. Most models agree with each other to within a factor of a few, indicating that the predictions are robust. Of the lines we consider, C iii (977 Å) is the strongest line, but it typically traces gas colder than 105 K. The same is true for Si iv (1393,1403 Å). The second strongest line, C iv (1548,1551 Å), traces circumgalactic gas with T∼ 105 K. O vi (1032,1038 Å) and Ne viii (770,780 Å) probe the warmer (T∼ 105.5 and 106 K, respectively) and more diffuse gas that may be a better tracer of the large-scale structure. N v (1239,1243 Å) 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 (surface brightness ≳103 photon s−1 cm−2 sr−1) comes from relatively dense (ρ > 102ρmean) and metal rich (Z≳ 0.1 Z⊙) 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 on to or flowing out of galaxies and groups.