各种坐标系

HSR

A useful rest frame for objects in the solar neighbourhood is the so-called barycentric standard-of-rest (BSR) frame which uses the barycentre of the Solar System as reference point. Normally, the spectra observed with a radio telescope are already provided in the BSR frame. The BSR frame is often referred to as the heliocentric standard-of-rest (HSR) frame. The latter one, however, uses the barycentre of the Sun as reference point instead of the Solar System barycentre. The difference between barycentric and heliocentric velocities, however, is rather small and negligible in most cases.

LSR

For objects located in the Galaxy at larger distances from the Sun one usually uses the local standard-of-rest (LSR) frame as the reference for radial velocities. The LSR frame accounts for the peculiar motion of the Sun of about 16.55 km/s with respect to the regular rotation of the Galaxy. Radial velocities in the LSR frame can be calculated from barycentric velocities via vLSR = vBSR + 9 cos(l) cos(b) + 12 sin(l) cos(b) + 7 sin(b)

where l and b are the Galactic longitude and latitude. This definition is the so-called “dynamical defintion” (also referred to as the LSRD) as specified by the IAU. There is an alternative “kinematical definition” (referred to as LSRK) which results in a slightly higher velocity of about 20 km/s in the direction of (α,δ) = (270°,30°) in the B1900 system. However, the LSRD definition is the one most commonly used and usually referred to as the LSR.

GSR

For the description of circumgalactic objects it is useful to correct also for the rotation of our Milky Way of 220 km/s. The corresponding reference frame, the so-called Galactic standard-of-rest (GSR) frame, is derived from the LSR frame via vGSR = vLSR + 220 sin(l) cos(b)

LGSR

For objects spread across the Local Group a reference frame accounting for the motion of our Milky Way of about 80 km/s with respect to the Local Group barycentre would be ideal. The corresponding radial velocities in the so-called Local Group standard-of-rest (LGSR) frame can be calculated from the GSR velocities via vLGSR = vGSR − 62 cos(l) cos(b) + 40 sin(l) cos(b) − 35 sin(b)