Supergalactic plane

The Supergalactic plane (SGP) is a plane roughly perpendicular to the Galactic plane of the Milky Way. It represents a planar structure in the local distribution of galaxies. The Milky Way is also located within the Supergalactic Plane.

Supergalactic and galactic plane

The distinction of the Supergalactic plane lies in the observations of it being filled with more proportion of one type of galaxies (elliptical galaxy) while the other main type of galaxies (disc galaxy - like our Milky Way with spiral arms) is largely missing from this Supergalactic Plane.

Importance

Differences in galaxies distribution

Hammer projection in supergalactic coordinates of the distribution of the most massive disks (left), intermediates (centre) and ellipticals (right) in the redshift range of 0.01 < z < 0.02, as reported in the 2MRS survey (top) and as modelled in SIBELIUS (bottom). The shaded areas correspond to |sin(SGB)| <0.2(−11.5⁰ ≤ SGB ≤ 11.5⁰) around the supergalactic equator. In both the 2MRS data and the SIBELIUS simulation, a significantly higher fraction of massive ellipticals than of massive disks lie close to the supergalactic plane.

Galaxies are observed to be distributed non-uniformly in our universe.

This fact has been recently (2023) validated in a research by Durham University and University of Helsinki which compared the predicted and observed distributions of galaxies using the Two Micron All Sky Survey (2MASS) galaxy redshift catalogue of actual telescope observations vis-a-vis supercomputer simulation using their SIBELIUS (Simulations Beyond the Local Universe).^[1]

The research findings concluded that different distributions of elliptical and disc galaxies is occurring due to different environments inside and outside the Supergalactic Plane.^[2]

As per the research findings, the galaxies tend to interact and merge more frequently and easily within the Supergalactic Plane and hence, the spiral galaxies also change into elliptical galaxies - while outside the Supergalactic Plane, the spiral galaxies retain their spiral structures due to their relative isolation.

Possible clustering of UHECR along SGP

The Pierre Auger Observatory is conducting studies to confirm correlation of ultra-high-energy cosmic rays (UHECRs) vis-a-vis the Supergalactic plane in the southern sky, whereas the Telescope Array Project (TA) does the same in the northern sky.^[3]

In another effort, the IceCube Neutrino Observatory, the ANTARES collaboration, the Pierre Auger Observatory and the Telescope Array (TA) collaboration data are also being analysed together to possibly correlate neutrino events and UHECRs.

CMB dipole

Many massive galaxy superclusters like Virgo, Hydra–Centaurus (Great Attractor), Norma, Perseus–Pisces, Coma, Hercules, Shapley, Ophiuchus lie along the direction of the observed CMB dipole - and most of these clusters like Virgo, Hydra-Centaurus etc. comprise the Supergalactic plane or are usually referenced w.r.t this Supergalactic plane.

Extent and Orientation

Galaxies and galaxy clusters < 50 Mly away from Earth plotted in the supergalactic plane

The orientation of this plane depends on the radius at which galaxies are being sampled.^[4]

The SGP is observed and considered to be a somewhat uniform or homogenous plane largely upto 40 h^-1 Mpc (or about 28 Mpc if dimensionless hubble constant, h is considered = 0.7, or which equals approx 90 Mly) which implies roughly the size of the Local Volume of galaxies. However, the planar structure is observed to be more like a dumbbell at radius of 50 h^-1 Mpc where the Great Attractor and Perseus-Pisces superclusters take precedence.

For sake of comparison, the Local Group of galaxies which includes our Milky Way and also the Andromeda Galaxy is about 5 Mpc in diameter.

Difference vs Extragalactic

Extragalactic is concerned with objects outside the Milky Way galaxy - whereas study of the Milky Way galaxy and all its contents (which has a diameter of approx 26,800 parsecs (26.8 Kpc or 87,400 ly)^[5][a]) is referred to as galactic.

See also

• Supergalactic coordinate system
• Galactic plane

Notes

1. This is the diameter measured using the D₂₅ standard. A 2018 study suggested that there is a presence of disk stars beyond this diameter, although it is not clear how much of this influences the surface brightness profile.^[6][7]