Detection of the Warm-Hot Intergalactic Medium in Emission

The Warm Hot Intergalactic Medium is a tenuous X-ray emitting filamentary plasma that is predicted to contain a considerable fraction of the unaccounted baryons matter in the Universe. It is believed to fill the spaces between galaxies and contribute to the Diffuse X-ray Background and its temperature and density make it invisible to all but low energy X-ray and UV observations, mostly through excitation lines of highly ionized heavy elements. The detection of the warmer filaments has been pursued for decades, but since their emission is often blurred with other X-ray background sources, no uncontroversial/conclusive detection has ever been reported. We performed the first X-ray detection of the Warm Hot Intergalactic Medium in emission at an unprecedented confidence level using power spectrum analysis. Thanks to wide-field XMM-Newton and eROSITA data, we were able to isolate its contribution from that of other sources modeled in previous studies using the same method. The measured contribution from WHIM filaments isolated in this work is in very good agreement with numerical simulations. Our result fills a gap in our picture of the large-scale structure of the Universe, in which this filamentary gas plays a fundamental role in the interaction and co-evolution with both the visible cosmic structures and the underlying dark matter distribution.