Have the missing cosmic baryons been found?

TL;DR

This paper presents evidence that a significant portion of the universe's missing baryons are likely contained in a hot, diffuse intergalactic medium, as indicated by uniform soft X-ray absorption observed in high-redshift sources.

Contribution

It provides observational evidence supporting the existence of the missing baryons in a hot intergalactic medium through soft X-ray absorption measurements.

Findings

Soft X-ray absorption consistent with hot IGM

Absorption magnitude and redshift dependence match predictions

Supports the hypothesis of missing baryons in a diffuse medium

Abstract

The angular power spectrum and polarization of the cosmic microwave background radiation (CMB), the relative abundances of primordial hydrogen, deuterium and helium isotopes, and the large-scale structure of the universe all indicate that 4.5% of the current mass density of the universe consists of baryons. However, only a small fraction of these baryons can be accounted for in stars and gas inside galaxies, galaxy groups and galaxy clusters, and in spectral-line absorbing gas in the intergalactic medium (IGM). Too hot to show up in Lyman-absorption, too cool to cause detectable spectral distortions of the cosmic microwave background radiation, and too diffused to emit detectable X-rays, about 90% of the cosmic baryons remain missing in the local universe (redshift z~0). Here, we report on prevalent, isotropic, source independent, and fairly uniform soft X-ray absorption along the lines of sight to high-z gamma-ray bursts (GRBs) and quasars. It has the magnitude, redshift and energy dependence that are expected from a hot IGM that contains the missing cosmological baryons and has a mean metallicity similar to that in the intracluster medium (ICM) of galaxy clusters.