Peculiar velocities

Peculiar motion or peculiar velocity refers to the velocity of an object relative to a rest frame — usually a frame in which the average velocity of some objects is zero.

In galactic astronomy, peculiar motion refers to the motion of an object (usually a star) relative to a Galactic rest frame.

Local objects are often related in terms of proper motion and radial velocity, which can be combined through vector addition to produce the object's motion relative to the Sun. Velocities for local objects are sometimes reported with respect to the local standard of rest (LSR) — the average local motion of material in the galaxy — instead of the Sun's rest frame. Translating between the LSR and heliocentric rest frames requires the calculation of the Sun's peculiar velocity in the LSR.

In physical cosmology, peculiar velocity refers to the components of a galaxy's velocity that deviate from the Hubble flow. According to Hubble's Law, galaxies recede from us at speeds proportional to their distance from us.

Galaxies are not distributed evenly throughout observable space, but are typically found in groups or clusters, where they have a significant gravitational effect on each other. Velocity dispersions of galaxies arising from this gravitational attraction are usually in the hundreds of kilometers per second, but they can rise to over 1000 km/s in rich clusters. This velocity can alter the recessional velocity that one would expect from the Hubble flow and effect the observed redshift of objects via the relativistic Doppler effect. The Doppler redshift due to peculiar velocities is

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