halotools.empirical_models.delta_vir(cosmology, redshift)[source] [edit on github]

The virial overdensity in units of the critical density, using the fitting formula of Bryan & Norman 1998, assuming \(\Omega_{\Lambda} = 0.\)

\(\Delta_{\rm vir}(x) = 18\pi^{2} + 82x - 39x^{2}\), where \(x \equiv \Omega_{m}(z)-1\).

See Halo Mass and Radius Definitions for details.

cosmology : object

Instance of an Astropy cosmology object.

redshift: array_like

Can be a scalar or a numpy array.

delta: array_like

The virial overdensity. Has the same dimensions as the input redshift.

See also

The threshold density for a given mass definition.