# Units¶

As much as possible, all functions in Colossus use a consistent unit system. The only quantity represented by two different units in Colossus is length: distances in the halo module are represented as physical kpc, whereas distances in the cosmology module are represented as comoving Mpc. This distinction is necessary because halos are collapsed objects that do not “feel” the expansion of the universe any more, whereas cosmology is most naturally represented in comoving units. For completeness, the following table lists all units used by each top-level module:

Variable

Cosmology

Large-scale str.

Dark matter halos

Length

Comoving $${\rm Mpc}/h$$

Comoving $${\rm Mpc}/h$$

Physical $${\rm kpc}/h$$

Wavenumber

Comoving $$h/{\rm Mpc}$$

Comoving $$h/{\rm Mpc}$$

Comoving $$h/{\rm Mpc}$$

Time

Gigayears

Gigayears

Mass

$$M_{\odot}/h$$

$$M_{\odot}/h$$

$$M_{\odot}/h$$

Density

Physical $$M_{\odot} h^2 / {\rm kpc}^3$$

Physical $$M_{\odot} h^2 / {\rm kpc}^3$$

Physical $$M_{\odot} h^2 / {\rm kpc}^3$$

Surface density

Physical $$M_{\odot} h / {\rm kpc}^2$$

Note that, even in the cosmology module, density units are based on physical kpc. Thus, quantities such as the mean density of the universe are compatible with the densities computed in the halo module.

Physics modules

Cosmology