Rvir

Rvir is the virial radius, defined as the radius of a spherical region around a gravitationally bound object (most often a dark matter halo) within which the average density is a specified multiple of a reference density. The radius is used to characterize the size and mass of halos in cosmology and simulations. The quantity Mvir is the mass enclosed within Rvir.

Common conventions include R200c, where the mean interior density is 200 times the critical density, rho_crit(z), and R200m, where the mean interior density is 200 times the mean matter density, rho_m(z). A more general form uses Delta(z), the virial overdensity from the spherical collapse model, so that Mvir = (4/3) pi Rvir^3 Delta(z) rho_ref(z). In Lambda-CDM cosmologies, Delta(z) is not strictly constant and varies with redshift.

In practice, Rvir is estimated in simulations by locating the boundary where the halo transitions from bound, high-density material to the less bound environment; many halo finders report Rvir and Mvir, or R200c/M200c, etc. Observationally, Rvir is not directly measurable; estimates rely on kinematics (rotation curves, velocity dispersion), X-ray temperatures, lensing, or hydrostatic equilibrium assumptions.

Typical scales vary by mass: galaxy-sized halos have Rvir of order 100–200 kiloparsecs; cluster-sized halos have Rvir of order 1–2 megaparsecs. The concept provides a convenient, though model-dependent, way to compare halo sizes across studies.