cluster radius
(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.
Estimated proportions:
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 - 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.
(galaxy clusters,measure,radius)
Further reading:
https://ui.adsabs.harvard.edu/abs/2015ApJ...810...36M/abstract
http://www.physics.utah.edu/snowcluster/archive/2018/talks/Nagai.pdf
https://ned.ipac.caltech.edu/level5/Sept09/Bohringer/Bohringer3.html
Index