Supergalactic coordinate system

In the 1950s the astronomer Gérard de Vaucouleurs recognized the existence of a flattened “local supercluster” from the Shapley-Ames Catalog in the environment of the Milky Way. He noticed that when one plots nearby galaxies in 3D, they lie more or less on a plane. A flattened distribution of nebulae had earlier been noted by William Herschel. Vera Rubin had also identified the supergalactic plane in the 1950s, but her data remained unpublished.[1] The plane delineated by various galaxies defined in 1976 the equator of the supergalactic coordinate system he developed. In years thereafter with more observation data available de Vaucouleurs findings about the existence of the plane proved right.

Based on the supergalactic coordinate system of de Vaucouleurs, surveys[2] in recent years determined the positions of nearby galaxy clusters relative to the supergalactic plane. Amongst others the Virgo cluster, the Norma cluster (including the Great Attractor), the Coma cluster, the Pisces-Perseus supercluster, the Hydra cluster, the Centaurus cluster, the Pisces-Cetus supercluster and the Shapley Concentration were found to be near the supergalactic plane.

The supergalactic coordinate system is a spherical coordinate system in which the equator is the supergalactic plane.

By convention, supergalactic latitude is usually abbreviated SGB, and supergalactic longitude as SGL, by analogy to b and l conventionally used for galactic coordinates.

Supergalactic and Ecliptic plane

• The zero point [3] is where the supergalactic plane intersects with the galactic plane - (SGB = 0°, SGL = 0°) - this lies at (l_x = 137.37°, b_x = 0°). In J2000 equatorial coordinates, this is approximately 2.82^h, +59.5°.
• The plane passes through Earth, because the supergalactic plane is identified as a plane observed from Earth.
• The north supergalactic pole (SGB = 90°) lies at galactic coordinates in the constellation of Hercules (l_z = 47.37°, b_z = +6.32°). In the equatorial coordinate system (epoch J2000), this is approximately RA = 18.9^h, Dec = +15.7°.

The transformation from a triple of Cartesian supergalactic coordinates to a triple of galactic coordinates is

${\displaystyle {\begin{bmatrix}x\\y\\z\end{bmatrix}}_{\text{gal}}={\begin{bmatrix}\cos l_{x}\cos b_{x}&\sin l_{z}\cos b_{z}\sin b_{x}-\sin b_{z}\sin l_{x}\cos b_{x}&\cos l_{z}\cos b_{z}\\\sin l_{x}\cos b_{x}&\sin b_{z}\cos l_{x}\cos b_{x}-\cos l_{z}\cos b_{z}\sin b_{x}&\sin l_{z}\cos b_{z}\\\sin b_{x}&\cos b_{z}\cos b_{x}\sin \left(l_{x}-l_{z}\right)&\sin b_{z}\end{bmatrix}}{\begin{bmatrix}x\\y\\z\end{bmatrix}}_{\text{superg}}}$

Supergalactic and Galactic plane

The left column in this matrix is the image of the origin of the supergalactic system in the galactic system, the right column in this matrix is the image of the north pole of the supergalactic coordinates in the galactic system, and the middle column is the cross product (to complete the right-handed coordinate system).

• Coordinate system
• Celestial coordinate system

• Lahav, O.; Santiago, B.X. (2000). "The supergalactic plane revisited with the Optical Redshift Survey". MNRAS. 312 (1): 166–176. arXiv:astro-ph/9809343. Bibcode:2000MNRAS.312..166L. doi:10.1046/j.1365-8711.2000.03145.x. S2CID 17951943.
• Theuns, Tom. "Nearest group of galaxies". Institute for Computational Astronomy. icc.dur.ac.uk (course lecture notes). Durham University. This is a map of galaxies that lie within 20 million light years plotted onto the supergalactic plane.
• Fisher, K.A. (17 July 2006). "Understanding the supergalactic coordinates" (pers. site).
• "The universe beyond 10 megaparsecs in Supergalactic coordinates".
• Tully, R.B. (1989). "Support for three controversial claims made by Gérard de Vaucouleurs". In Corwin, H.G.; Bottinelli, L. (eds.). The World of Galaxies. New York, NY: Springer. pp. 408–419. doi:10.1007/978-1-4613-9356-6_62. ISBN 978-1-4613-9356-6.