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 2 smaller than that 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% ± 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 (χ²>136, N = 19). The error bar in the X-ray flux is dominated, instead, by cosmic variation and model uncertainties.