Shapley Supercluster

From Wikipedia, the free encyclopedia

Map of the Shapley Supercluster
Map of the Shapley Supercluster

The Shapley Supercluster is the largest concentration of galaxies in our nearby Universe that forms a gravitationally interacting unit, thereby pulling itself together instead of expanding with the Universe. It appears as a striking overdensity in the distribution of galaxies in the constellation of Centaurus, approximately 650 million light years from the Milky way.

In recent times, the Shapley supercluster was re-discovered by Somak Raychaudhury, from a survey of galaxies from UK Schmidt Telescope Sky survey plates, using the Automated Plate Measuring Facility (APM) at the University of Cambridge in England. In this paper, the Supercluster was named after Harlow Shapley, in recognition of his pioneering survey of galaxies in which this concentration of galaxies was first seen. Around the same time, Roberto Scaramella and co-workers had also noticed a remarkable concentration of clusters in the Abell catalogue of clusters of galaxies: they had named it the Alpha concentration.

[edit] History: Shapley's galaxy survey

In the late 1920s, Harlow Shapley and his colleagues at the Harvard College Observatory started a survey of galaxies in the southern sky, using photographic plates obtained at the 24-inch Bruce telescope at Bloemfontein, South Africa. By 1932, Shapley reported the discovery of 76,000 galaxies brighter than 18th apparent magnitude in a third of the southern sky, based on galaxy counts from his plates. Some of this data was later published as part of the Harvard galaxy counts, intended to map Galactic obscuration and to find the space density of galaxies.

In this catalog, Shapley could see most of the `Coma-Virgo cloud' (now known to be a superposition of the Coma Supercluster and the Virgo supercluster), but found a `cloud' in the constellation of Centaurus to be the most striking concentration of galaxies. He found it particularly interesting because of its great linear dimension, the numerous population and distinctly elongated form. This can be identified with what we now know as the core of the Shapley Supercluster. Shapley estimated the distance to this cloud to be 14 times that to the Virgo cluster, from the average diameters of the galaxies.

[edit] Current interest

The Shapley Supercluster lies very close to the direction in which the Local Group of galaxies (including our Galaxy) is moving with respect to the rest frame of the Universe defined by the Cosmic microwave background radiation. This has led many to speculate that the Shapley Supercluster may indeed be one of the major causes of our peculiar motion (the Great Attractor), and has led to a surge of interest in this Supercluster.

[edit] External links