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Local Supercluster

supercluster

Local supercluster. Each white dot is a galaxy; red regions have lots of galaxies, dark blue regions are voids with few. The white lines represent flow streams, along which galaxies are moving toward the center of mass of Laniakea. The blue dot to the right is our location, near the edge of the supercluster.
Illustration by SDvision interactive visualization software by DP at CEA/Saclay, France.

Local Supercluster

Our Milky Way galaxy is a member of the Local Group of galaxies which lies on the outskirts of a large structure of galaxies now called the “Local Supercluster.” Although it was first identified on very early sky maps of the distribution of nebulae (e.g. Messier’s catalog of nebulae and the New General Catalogue of Dreyer), it was only first recognized as a dynamical entity about 50 years ago by Gerard de Vaucouleurs after early measurements of the redshifts of its brightest members confirmed that they were generally all in the same volume of space and relatively nearby. DeVaucouleurs (1956) originally called the structure the “Local Supergalaxy,” but in 1958 he renamed it the Local Supercluster (LSC). We now know of many structures similar or larger in size, often containing more than one galaxy cluster, and generally called Superclusters.

The LSC is roughly centered on the Virgo Cluster, the nearest cluster of galaxies containing several hundred bright galaxies in a volume a few Megaparsecs (roughly 10 million light-years) across. As can be seen by looking at the sky map below, it is a flattened, planar structure with a halo of other groups and clouds of galaxies. The Virgo Cluster itself, subject of another exercise, is at a distance of approximately 16.7 Megaparsecs (55 million light-years) as determined by distance measurements with the Hubble Space telescope.

About 30 years ago, with the discovery of the ubiquitousness of dark matter and the realization that the LSC should have a measurable effect on the Local Group (Peebles 1973; Silk 1973; Gunn)

The purpose of this project is to study the properties of the Local Supercluster with a new and deep all-sky survey of galaxies, the 2MASS Redshift Survey (2MRS). We will measure several global properties of the LSC, including its size and shape. We will examine maps of the distribution of galaxies in the supercluster and the distribution by morphological type.

Local Supercluster

The Local Supercluster, a flattened collection of about 100 groups and clusters of galaxies including the Local Group. The Local Supercluster is centred approximately on the Virgo cluster and has a total extent of roughly 200.000.000 light-years. Its precise boundaries, however, are difficult to define inasmuch as the local enhancement in numbers of galaxies above the cosmological average in all likelihood just blends smoothly into the background.

Also apparent in the Shapley-Ames maps were three independent concentrations of galaxies, separate superclusters viewed from a distance. Astronomers now believe superclusters fill perhaps 10 percent of the volume of the universe. Most galaxies, groups, and clusters belong to superclusters, the space between superclusters being relatively empty. The dimensions of superclusters range up to a few times 100.000.000 light-years. For larger scales the distribution of galaxies is essentially homogeneous and isotropic—that is, there is no evidence for the clustering of superclusters. This fact can be understood by recognizing that the time it takes a randomly moving galaxy to traverse the long axis of a supercluster is typically comparable to the age of the universe. Thus, if the universe started out homogeneous and isotropic on small scales, there simply has not been enough time for it to become inhomogeneous on scales much larger than superclusters. This interpretation is consistent with the observation that superclusters themselves look dynamically unrelaxed—that is, they lack the regular equilibrium shapes and central concentrations that typify systems well mixed by several crossings.

 

LOCAL SUPERCLUSTER

The locations of the galaxies within the volume of the Local Supercluster are displayed here on an Aitoff equal area grid in celestial coordinates, Right Ascension (R.A.) and Declination (Decl.).

All of these objects have measured recessional velocities less than 3,000 km/s.

The map is centered on R.A.= 6 hours, Decl. = 0 degrees.

The continous lines that rise diagonally through the center of the map show the locus in celestial coordinates of the lines of constant galactic latitude at -20deg, 0 and +20 deg. delineating the Zone of Avoidance, where distant galaxies are obscured at optical wavelengths by the dust and gas within the Milky Way.

It is pretty clear on this representation that there are more galaxies on the left side of the map than on the right side. This asymmetry occurs because the Local Group is located on the edge of the Local Supercluster. When we look towards the center, we see lots of galaxies; when we look away from the center, we see many fewer.

With a bit more imagination, trace a continuous line through the concentration in Virgo, across the zone of avoidance, through the right side and back again through the north pole. This continuous distribution is the Supergalactic plane. Although less well defined that the galactic plane, its presence indicates that the Local Supercluster, like the Milky Way, is a flattened structure.

Within the Local Supercluster, galaxies tend to be found in groups and clouds, with other regions being relatively empty.

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