Where are the missing baryons in clusters?

TL;DR

This paper proposes that the so-called missing baryons in galaxy clusters are actually present but located in the outer regions beyond the observed central areas, explaining the apparent deficiency.

Contribution

The study demonstrates that baryons are more extended than previously observed, and the baryon fraction reaches the cosmic value near the virial radius when extrapolated.

Findings

Baryon fraction increases with radius in clusters.

Baryons are likely in the outskirts of clusters, not missing.

Heating processes cause gas expansion, explaining the distribution.

Abstract

Observations of clusters of galaxies suggest that they contain significantly fewer baryons (gas plus stars) than the cosmic baryon fraction. This `missing baryon' puzzle is especially surprising for the most massive clusters which are expected to be representative of the cosmic matter content of the universe (baryons and dark matter). Here we show that the baryons may not actually be missing from clusters, but rather are extended to larger radii than typically observed. The baryon deficiency is typically observed in the central regions of clusters (~0.5 the virial radius). However, the observed gas-density profile is significantly shallower than the mass-density profile, implying that the gas is more extended than the mass and that the gas fraction increases with radius. We use the observed density profiles of gas and mass in clusters to extrapolate the measured baryon fraction as a function of radius and as a function of cluster mass. We find that the baryon fraction reaches the cosmic value near the virial radius for all groups and clusters above 5e13 solar masses. This suggests that the baryons are not missing, they are simply located in cluster outskirts. Heating processes (shock-heating of the intracluster gas, plus supernovae and AGN feedback) that cause the gas to expand are likely explanations for these results. Upcoming observations should be able to detect these baryons.