Why is the gas between galaxies in a cluster so hot?

As gas falls into the deep gravitational well of a cluster, it is compressed and shock-heated, reaching tens of millions of degrees. At these temperatures it is fully ionized and glows in X-rays.

What is the cooling flow problem?

In dense cluster cores the gas radiates energy fast enough that it should cool and form many stars, yet observations find little such cooling. The resolution is that energy injected by the central black hole reheats the gas, preventing the predicted cooling flow.

The Intracluster Medium

The space between galaxies in a cluster is filled with hot, X-ray-emitting plasma whose mass exceeds that of all the cluster's stars combined.

Definition

The intracluster medium is the diffuse, hot ionized gas, at temperatures of tens of millions of degrees, that fills the volume between galaxies in a cluster, emitting X-rays and constituting the dominant baryonic component of the cluster.

Scope

This topic covers the properties of the hot intracluster gas, its X-ray emission and the diagnostics it provides, the question of cooling flows in cluster cores and their suppression by feedback, the chemical enrichment of the gas, and the Sunyaev-Zeldovich effect by which clusters imprint on the cosmic microwave background.

Core questions

What are the temperature, density, and composition of the intracluster gas?
How does X-ray emission diagnose the state of the gas?
Do cluster cores undergo cooling flows, and what prevents runaway cooling?
How does the Sunyaev-Zeldovich effect let clusters be detected?

Key theories

Hot X-ray-emitting gas: The intracluster medium is plasma heated by the cluster's gravitational potential to X-ray-emitting temperatures, its emission revealing the gas density, temperature, and total cluster mass.
Cooling flows and feedback: Dense cluster cores should cool rapidly and form stars, but observations show far less cooling than expected, implying heating, primarily from the central active galactic nucleus, that offsets the radiative losses.
The Sunyaev-Zeldovich effect: Hot electrons in the intracluster medium scatter cosmic microwave background photons to higher energies, producing a distortion that detects clusters independently of distance.

Clinical relevance

The intracluster medium holds most of a cluster's ordinary matter and records its history of heating and enrichment; its X-ray and Sunyaev-Zeldovich signatures are primary tools for finding clusters and measuring their masses for cosmology.

History

X-ray satellites in the 1970s revealed that clusters are luminous X-ray sources, establishing the hot intracluster medium. The expected cooling flows, reviewed by Fabian in 1994, proved largely absent in later high-resolution data, pointing to AGN feedback, while the Sunyaev-Zeldovich effect became a powerful cluster-finding technique.

Key figures

Craig Sarazin
Andrew Fabian
Rashid Sunyaev
Yakov Zeldovich

Seminal works

sarazin1988
fabian1994
sunyaev1972

Frequently asked questions

Why is the gas between galaxies in a cluster so hot?: As gas falls into the deep gravitational well of a cluster, it is compressed and shock-heated, reaching tens of millions of degrees. At these temperatures it is fully ionized and glows in X-rays.
What is the cooling flow problem?: In dense cluster cores the gas radiates energy fast enough that it should cool and form many stars, yet observations find little such cooling. The resolution is that energy injected by the central black hole reheats the gas, preventing the predicted cooling flow.