cluster radius

A galaxy cluster's radius (cluster radius) is not distinct, and various criteria are used to define one, such as the extent of the extra mass density represented by the cluster. Some measures that are used:

• R_200m - radius covering a volume with mass density 200 times the average of the universe.
• R_200c (or just R₂₀₀) - radius covering a volume with mass density 200 times the critical density at the cluster's redshift.
• R_500c (R₅₀₀) - similar for 500 times.
• R_sp - splashback radius, i.e., the radius to which to which matter that has fallen into the halo again reaches before being pulled back in again.

The mass in question includes the cluster's intracluster medium and dark matter halo. Estimated proportions of these measures:

R500c:R200c:R200m:Rsp≈1:1.4:3:4

Distant galaxy clusters are often scaled in relation to the critical density at the redshift of the cluster, because there are galaxy cluster attributes that follow that scale. The commonly-used galaxy cluster radius value R₅₀₀ indicates the radius that surrounds a volume whose mean density is 500 times the critical density at that redshift. Radii within the cluster can be usefully scaled as R/R₅₀₀. Values used, based, e.g., on R₅₀₀:

• M₅₀₀ - mass of the cluster within R₅₀₀.
• N₅₀₀ - number of galaxies within the same volume.
• Y₅₀₀ - Compton scattering Y-parameter integrated over the same volume.
• L₅₀₀ - luminosity over the same volume.
• P₅₀₀ - a pressure scaled to R₅₀₀.

Other parameters can be defined similarly.