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Concept
Peculiar velocity
Summary
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 commonly examined as to their vectors of position angle and radial velocity. These can be combined through vector addition to state 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 one on another. 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 would be expected from the Hubble flow and affect the observed redshift of objects via the relativistic Doppler effect. The Doppler redshift due to peculiar velocities is
which is approximately
for low velocities (small redshifts). This combines with the redshift from the Hubble flow and the redshift from our own motion to give the observed redshift
(There may also be a gravitational redshift to consider.)
The radial velocity of a cosmologically "close" object can be approximated by
with contributions from both the Hubble flow and peculiar velocity terms, where is the Hubble constant and is the distance to the object.
Official source
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