How the diffuse Universe cools

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 Overwhelmingly Large Simulations 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 asymptotic giant branch stars and Type Ia supernova 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, FeII and S III. The great majority of energy is emitted in the ultraviolet (UV) band (lambda = 100-4000 angstrom), 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 characterization of diffuse UV emission.