||This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. (January 2016) (Learn how and when to remove this template message)|
Peculiar motion or peculiar velocity refers to the true velocity of an object, relative to a rest frame.
Local objects are usually related in terms of proper motion and radial velocity, that is the amount an object appears to move across the sphere of the sky, and the measured speed at which it travels towards or away from us, respectively. Proper motion and radial velocity can be combined through vector addition to produce the object's motion relative to the Sun. For "peculiar" motion, the local standard of rest used is not usually the Sun but instead the average motion of a chosen set of local stars.
According to Hubble's law, galaxies recede from us at a speed proportional to their distance. The relationship between speed and distance would be exact in the absence of other effects.
Galaxies are not distributed evenly throughout observable space, but typically found in groups or clusters, ranging in size from fewer than a dozen to several thousands. All these nearby galaxies have a gravitational effect, to the extent that any one galaxy can have a velocity of over 1,000 km/s in an apparently random direction. This velocity can therefore add to, or subtract from, the radial velocity that one would expect from the Hubble flow.
The main consequence is that, in determining the distance of a single galaxy, a possible error must be assumed. This error becomes smaller, relative to the total speed, as distance increases. An estimate can be refined by taking the average velocity of a group of galaxies: their peculiar velocities, assumed to be essentially random, will cancel each other, leaving a much more accurate measurement.