Evidence for the Missing Baryons in the Angular Correlation of the Diffuse X-ray Background

Abstract

The amount of detected baryons in the local Universe is at least a factor of two smaller than measured at high redshift. It is believed that a significant fraction of the baryons in the current Universe is "hiding" in a hot filamentary structure filling the intergalactic space, the Warm-Hot Intergalactic Medium ($WHIM$). We found evidence of the missing baryons in the $WHIM$ by detecting their signature on the angular correlation of diffuse X-ray emission with the XMM-Newton satellite. Our result indicates that $(12\pm 5)$% of the total diffuse X-ray emission in the energy range 0.4-0.6 keV is due to intergalactic filaments. The statistical significance of our detection is several sigmas ($χ^2>136$ N=19). The error bar in the X-ray flux is dominated, instead, by cosmic variation and model uncertainties.