(radius of a galaxy cluster)
A galaxy cluster'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).
It is a virial radius. Some that are used:
- 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.
- R500c (R500) - similar for 500 times.
- Rsp - splashback radius, i.e., the radius to which matter that has fallen through the halo reaches.
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 over the same volume.
- P500 - a pressure scaled to R500.
Other parameters can be defined similarly.