Galaxy Clusters
A galaxy cluster is the largest class of gravitationally bound structure in the universe: a system of hundreds to thousands of galaxies, with total masses of roughly 1014 to 1015 solar masses, in which most of the mass is dark matter, most of the ordinary matter is hot (107–108 K) X-ray emitting intracluster gas, and the galaxies themselves contribute only a few percent of the mass.
Curious
Galaxies are not scattered randomly. Gravity gathers them into groups and clusters, which in turn link into filaments and walls (see Large-Scale Structure). Our own Milky Way belongs to a small group, the Local Group, near the outskirts of the Virgo supercluster region.
Clusters were where evidence for dark matter first appeared. In 1933 Fritz Zwicky measured the speeds of galaxies in the Coma Cluster and found them moving far too fast to stay bound if only the visible matter held them. Modern X-ray and lensing observations confirm that clusters are mostly dark matter.
Exploring
| Component | Approximate share of cluster mass |
|---|---|
| Dark matter | ~80–85% |
| Hot intracluster gas | ~12–15% |
| Stars in galaxies | ~2–3% |
Deep Dive
Three independent mass probes. Cluster masses can be derived from (1) galaxy velocity dispersions via the virial theorem, (2) the temperature and density of the X-ray gas, assuming hydrostatic equilibrium (as for stars; see Stellar Structure), and (3) gravitational lensing (see Gravitational Lensing). They broadly agree, which is strong evidence that the unseen mass is real; hydrostatic masses are typically somewhat lower than lensing masses, attributed to non-thermal pressure support, an active topic.
The Bullet Cluster. In 2006, Clowe and collaborators studied 1E 0657-56, where two clusters collided. The hot gas, which dominates the baryons, was slowed and displaced by the collision, but weak lensing showed that most of the mass stayed with the galaxies, passing through. This is often cited as direct evidence for collisionless dark matter and against modified gravity without dark matter; defenders of modified gravity contend that some residual unseen mass is still required, so the interpretation is not unanimous.
Cosmological use. The abundance of massive clusters as a function of redshift is sensitive to the matter density and to the growth of structure, so cluster counts constrain dark energy (see Dark Energy). The Sunyaev–Zel’dovich effect, a distortion of the cosmic microwave background by hot cluster gas, finds clusters at any redshift. Clusters also show a cooling-flow puzzle: the gas at the centre should cool and form stars far faster than seen, and feedback from the central black hole is the leading explanation.
Concept Relationship Map
Requires: dark matter, large-scale structure
Enables: cluster cosmology, strong and weak lensing studies
Commonly confused with: galaxy groups and superclusters
Sources
- Zwicky, F. (1933). Helvetica Physica Acta, 6, 110–127.
- Clowe, D., et al. (2006). “A Direct Empirical Proof of the Existence of Dark Matter.” The Astrophysical Journal Letters, 648, L109–L113.
- Voit, G.M. (2005). “Tracing cosmic evolution with clusters of galaxies.” Reviews of Modern Physics, 77, 207–258.