Searching for the Missing Baryons in the Warm-Hot Intergalactic Medium

TL;DR

This paper reviews the challenges and prospects of detecting the elusive Warm-Hot Intergalactic Medium (WHIM), which is believed to contain a significant portion of the universe's missing baryons, using current and future X-ray observations.

Contribution

It critically assesses the current detection attempts of WHIM in absorption and discusses the potential of future X-ray missions for secure identification.

Findings

Detection of WHIM absorption features is at the limit of current XMM-Newton capabilities.

Controversial results highlight the difficulty of confirming WHIM presence.

Future X-ray missions are essential for proper characterization of the WHIM.

Abstract

At low redshift (z<2), almost half of the baryons in the Universe are not found in bound structures like galaxies and clusters and therefore most likely reside in a Warm-Hot Intergalactic Medium (WHIM), as predicted by simulations. Attempts to detect WHIM filaments at cosmological distances in absorption towards bright background sources have yielded controversial results that I review here. I argue that a secure detection of absorption features by the WHIM is at the limit of the XMM-Newton capabilities, but feasible. A proper characterisation of the whole WHIM belongs to the realm of future X-ray missions.