systemic velocity

The term systemic velocity is used for diffuse objects such as galaxies (especially), disks, or clouds, to indicate the velocity of the entire object (the "system"). It likely is meant to be the velocity of the object's center of mass, but determinations used are often some average of the velocities of its portions, or the velocity of a central portion. Also, according to specific usage, it may include or exclude the effects of Hubble expansion. Such a distinction in usage can be crucial, such as if a galaxy's cited systemic velocity were used as an approximation of its position within the Hubble flow, implying a certain distance. The distinction can also be moot, when the purpose of the systematic velocity is for comparison with other velocities such as that of some of the galaxy's stars or with its surrounding objects, as long as all the velocities follow the same convention.

The term systemic redshift is the redshift associated with an object's systemic velocity. The term is sometimes used when a galaxy's cosmological redshift determination is nontrivial, i.e., cases where a particular spectral line's peak is not a good representation.