  In this work we investigate the cooling channels of diffuse gas (i.e. n_H<0.1
cm^-3) in cosmology. We aim to identify the wavelengths where most of the
energy is radiated in the form of emission lines or continuum radiation, and
the main elements and ions responsible for the emission. We use a subset of
cosmological, hydrodynamical runs from the OWLS project to calculate the
emission of diffuse gas and its evolution with time. We find that at z=0 (z=2)
about 70 (80) per cent of the energy emitted by diffuse gas is carried by
emission lines, with the continuum radiation contributing the remainder.
Hydrogen lines in the Lyman series are the primary contributors to the line
emission, with a share of 16 (20) per cent. Oxygen lines are the main metal
contributors at high redshift, while silicon, carbon and iron lines are
strongest at low redshift, when the contributions of AGB stars and supernova Ia
explosions to the metal budget become important and when there is more hot gas.
The ionic species carrying the most energy are OIII, CII, CIII, SiII, SiIII,
FeII and SIII. The great majority of energy is emitted in the UV band
(lambda=100-4000 A), both as continuum radiation and line emission. With almost
no exception, all the strongest lines fall in this band. At high energies,
continuum radiation is dominant (e.g., 80 per cent in the X-ray band), while
lines contribute progressively more at lower energies. While the results do
depend on the details of the numerical implementation of the physical processes
modelled in the simulations, the comparison of results from different
simulations demonstrates that the variations are overall small, and that the
conclusions are fairly robust. Given the overwhelming importance of UV emission
for the cooling of diffuse gas, it is desirable to build instruments dedicated
to the detection and characterisation of diffuse UV emission.
