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 O III , C II , C III , Si II , Si III , Fe II and S III .
The great majority of energy is emitted in the UV band ( \lambda = 100 - 4000 Å ) , 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 modeled 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 .