Velocity Reference Frames Used in Radio Astronomy

-- | September 18, 2012

Question: I have read that radio astronomers are able to detect velocities in the objects they study and report these as V_LSR (LSR = local standard of rest). What are these velocities relative to? Are they relative to the observer on earth? — Karim

Answer: Before getting to the answer to your question, let me describe the more general problem of defining a reference point for velocities in radio astronomy. The observed radial velocity of an astronomical object (whether it be observed at radio wavelengths or not) is subject to a variety of projection effects. These include:

The rotation and the orbital motion of the Earth,
The motion of the Sun around the Galactic center,
The motion of our Galaxy within the Local Group.

An appropriate rest frame is required in order to be able to compare measurements made at different observatories and wavelengths. A useful rest frame for objects in the solar neighborhood uses the barycenter of the Solar System as a reference point, and is called the “Barycentric Standard of Rest” (BSR). A close cousin to the BSR is the “Heliocentric Standard of Rest” (HSR), which uses the barycenter of the Sun as its reference point (the difference between BSR and HSR is rather small.

For objects located beyond the Solar System, but still in the Milky Way Galaxy, one usually takes the local standard-of-rest (LSR) frame as reference for radial velocities. The LSR frame accounts for the peculiar motion of the Sun of 16.5 km/s with respect to the regular rotation of the Galaxy. For some simple equations that you can use to convert between BSR and LSR see Tobias Westmeier’s description of rest frames in astronomy.

Jeff Mangum

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