National Solar Observatory/Sacramento Peak
Supercluster

       Bigger  Smaller

Types: Filament   ---- Supercluster       ---- Galaxy Clouds

Size:  100 Mpc/h  ---- 50 Mpc/h           ---- 15 Mpc/h

Earth: (Filament) ---- Local Supercluster ---- Canes Venatici cloud

A supercluster is a group of galaxy clusters that seem associated with each other. Usually it is decided which galaxy clusters together form a supercluster by looking at the distances between the galaxy clusters. This method works well if the superclusters are clearly separated from each other, but if galaxy clusters can be found at all distances between the centers of two neighboring superclusters (as is often the case) then it may not be easy to decide whether a particular galaxy cluster about halfway between the supercluster centers belongs to the one or to the other supercluster.

The image shows the central one degree of the Virgo cluster of galaxies. All stars in the image are in our own galaxy and happen to lie in the same direction as the Virgo cluster.

Distances to galaxies and galaxy clusters are much more difficult to determine than the directions to those objects, so distances between such objects are usually much more accurate perpendicular to the radial direction than they are in the radial direction. This makes determination of which galaxy clusters belong to which superclusters more difficult, too.

A definition for superclusters that has less ambiguity would be a group of galaxy clusters that are bound together by gravity, just like the Sun, all the planets, and all other objects in the solar system are bound together by gravity. Such information is currently not known for most galaxy clusters, and it seems likely that not all galaxy clusters are bound by gravity to any one supercluster, so this definition also has its problems.

The Local Supercluster

Our galaxy lies in the Canes Venatici cloud of galaxies on the outskirts of the Local Supercluster, of which the Virgo galaxy cluster is the central part [2]. The Virgo galaxy cluster is the only rich galaxy cluster in the Local Supercluster, and none of the listed galaxy clouds has its center further from the Virgo cluster than our own. The major components (galaxy clusters) of the Local Supercluster are listed in the following table. "Name" is the common name of the cloud, "Dist" the distance of its center to us, "Size" its greatest extent, "Volume" its approximate volume, "N" the number of bright galaxies in the cloud (absolute magnitude brighter than -18 + 5 log h, which is about the estimated brightness of our own galaxy), and "Dens" the number of bright galaxies per unit volume. All distances are measured in Mpc/h (written Mpc in the table). The number of fainter galaxies in these galaxy groups is likely much greater than the number of bright galaxies.

Name	Dist	Size	Volume	N	Dens
	Mpc	Mpc	Mpc^3		Mpc^-3
Virgo Cluster	10.7	2.2	6	62	10	[APOD]
Canes Venatici	9.8	14.6	470	99	0.2
Virgo II (S)	11.6	10.6	200	55	0.3
Leo II	13.2	15.4	510	45	0.1
Virgo III	15.5	11.3	190	40	0.2
Crater (NGC 3672)	14.9	8.1	120	25	0.2
Leo I	7.8	5.4	20	15	0.6
Leo Minor (NGC 2841)	5.1	7.5	60	11	0.2
Draco (NGC 5907)	9.6	8.1	60	6	0.1
Antlia (NGC 2997)	7.5	4.6	40	5	0.1
NGC 5643	10.2	5	2	3	-
rest			~2900	8	0.003
Total			~4500	374	0.07

The Local Supercluster appears made of two major structures: a flattened disk which has a thickness of about 1 Mpc/h and contains about 60 % of the bright galaxies, and a roughly spherical halo which contains the remaining 40 % of the bright galaxies in a small number of clouds.

Nearby Superclusters

One recent survey of superclusters up to redshift z = 0.1 (about 300 Mpc/h) finds 130 superclusters [1]. The ones that are closer to us than 100 Mpc/h, those that contain at least 10 galaxy clusters, and those that have a proper name in the survey are listed below. In the table, "Id" means the identification in the survey, "Name" the proper name of the supercluster (usually made of the one or two constellations that the superclusters appears in, but the "Shapley" supercluster is named for its discoverer), "Dist" the distance of the center of the supercluster to us, in Mpc/h, and "Members" the number of rich galaxy clusters that are members of the supercluster.

Id	Name	Dist	Members
		Mpc/h
E1	Pegasus-Pisces A	261	4
E6	Pisces-Cetus	179	17
E8		82	5
E18	Cetus A	274	11
E20	Perseus	54	3
E27	Horologium-Reticulum	176	32
E41	Lepus	115	8
E53	Sextans	259	3
E56	Leo	94	8
E66	Ursa Majoris	215	3
E71		64	2
E74	Hydra-Centaurus	41	6
E80	Shapley	129	25
E83	Bootes	202	12
E90	Corona Borealis	212	10
E92	Hercules	104	10
E100		55	3
E108	Aquarius-Cetus	167	8
E109	Grus-Indus	218	8
E111	Aquarius-Capricornus	243	5
E122	Aquarius A	229	6
E123	Perseus-Pegasus A	120	4
E125	Pegasus-Pisces B	188	6
E126	Aquarius	260	12
E127	Perseus-Pegasus B	90	2
E128		86	2

References

1. "The structure of the Universe traced by rich clusters of galaxies", Einasto M., Einasto J., Tago E., Dalton G.B., Andernach H., Mon. Not. R. Astron. Soc. 269, 301 (1994) [ADS 1994MNRAS.269..301E]
2. "The Local Supercluster", R. Brent Tully, Astrophys. J., vol. 257, pp. 389-422 (1982) [ADS 1982ApJ...257..389T]

[LS 16 April 1997]