How Redshift and Metallicity Affect the Resonant Scattering X-Ray Emission from the Large-scale Structure of the Universe

Abstract The warm-hot intergalactic medium (WHIM), usually found in the filamentary large-scale structure of the Universe, is one of the contributors to the diffuse X-ray background in the soft (∼300–1000 eV) band. Its broadband emission is predominantly thermal, but resonant scattering of the background cosmic X-ray radiation (CXB) can enhance the amplitude of the emission lines of some ions by more than a factor ∼30. In this work, we explore how physical parameters (temperature, density, metallicity, and the redshift of the scattering structure) affect the magnitude of resonant scattering. This scattering is most effective for gas in the WHIM phase (at temperatures of the order of 10 6 eV and densities n H below 10 −4 cm −3 ), and at either lower temperatures or higher densities; resonant scattering becomes negligible in other phases. Metallicity directly affects the magnitude of resonant scattering, even if the amplitude of the lines does not increase linearly with metallicity. Since the CXB is more intense at higher redshifts, accordingly, the contribution of resonant scattering to the total emission is roughly twice as strong at z = 2 compared to z = 0.