Astrophysics (index)

Cluster Radius

(radius of a galaxy cluster)

A Galaxy Cluster (CL)'s radius (Cluster Radius) is measured according to various criteria, some based upon the extent of the extra mass density (more generally used for Dark Matter Halos):

  • R200m - radius covering a volume with mass density 200 times the average of the universe.
  • R200c (or just R200)- radius covering a volume with mass density 200 times the Critical Density at the cluster's Redshift (z).
  • R500c (R500)- similar for 500 times.
  • Rsp - Splashback Radius, i.e., the radius to which matter that has fallen through the halo reaches.

Estimated proportions:


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 - galaxies within the same volume.
  • Y500 - Compton Scattering Y-Parameter integrated over the same volume.
  • L500 - Luminosity (L) over the same volume.
  • P500 - a pressure scaled to R500.

Other parameters can be defined similarly.

(galaxy clusters,measure,radius)