Power Spectrum Analysis of the Warm Hot Intergalactic Medium

It is commonly accepted that the missing baryons in the Universe are hiding in the Warm Hot Intergalactic Medium (WHIM). Detecting and studying the WHIM remains one of the major goals of modern X-ray astrophysics, since its formation and evolution are tightly connected to the evolution of galaxies and structures.

The amount of baryons in the Universe has been measured using several methods. From high-redshift measurements it turns out that about 4% of the matter-energy density of the Universe is in the form of baryons. On the other hand, the results obtained from measurements in the local Universe show that this quantity is less than 2%. At the WHIM physical conditions the baryons are in the form of highly ionized plasma and therefore are invisible to all but low-energy X-ray and UV observations, mainly through excitation lines of highly ionized heavy elements and absorption features. Nevertheless, the WHIM contribution to the total emission in the X-ray energy band is weak, and the strength of the absorption features is feeble as well. in addition, the WHIM emission must be detected on top of a larger background, the Diffuse X-ray Background (DXB). In the soft X-ray band (below 1 keV) the DXB is the superposition of contributions from different sources that often have similar characteristics.

A promising way to detect the WHIM in emission is to use a statistical approach. The plasma filaments are expected to have a characteristic angular structure that can be studied with the power spectrum analysis, that has become the tool of choice in X-ray and other wavelengths investigations, since it allows to model different sources that contribute to the signal, letting their disentanglement. In this study we conduct a power spectrum analysis of the DXB with XMM-Chandra Deep Field South (CDFS), XMM-COSMOS, XMM-STRIPE82 and eROSITA Final Equatorial Depth Survey (eFEDS) in different energy bands, comparing the results to those obtained from numerical simulations.