Virgo cluster

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Image:Virgo.arp.750pix.jpg

The Virgo cluster is a cluster of galaxies, approximately 15 to 22 Mpc distant, comprising approximately 1300 (and possibly up to 2000) member galaxies. The cluster forms the heart of the larger Local supercluster, of which the Local Group is an outlying member.

Many of the brighter galaxies in this cluster, including the giant M87 were discovered in the late 1770's and early 1780's and subsequently included in Charles Messier's catalogue of non stellar objects. Described by Messier as nebulae without stars, their true nature wasn't recognized until the 1920's.

The cluster subtends a maximum arc of approximately 8 degrees centered in the constellation Virgo. Many of the member galaxies of the cluster are visible with a small telescope. The distance to the cluster is still an open question, with current best guesses, based on Cepheids using the Hubble Space Telescope, yielding an average of approximately 20 Mpc [1].

The cluster is a fairly heterogenous mixture of spirals and ellipticals. It is currently believed that the spirals of the cluster are distributed in a cigar-shaped prolate filament, approximately 4 times as long as wide, stretching along the line of sight from the Milky Way and connecting on its back end to the W cloud [2]. The ellipticals are concentrated much more than the spirals.

The cluster is an aggregrate of at least three separate subclumps centered on the galaxies M87, M86, and M49. Of the three subclumps, the one centered on M87 is the dominant one, with a mass of approximately 10¹⁴ solar masses, which is approximately an order of magnitude larger than the other two subclumps [3].

The large mass of the cluster is indicated by the high peculiar velocities of many of its galaxies, sometimes as high as 1,600 km/s with respect to the cluster's center.

The Virgo cluster lies within the Local supercluster, and its gravitational effects slow down the nearby galaxies. The large mass of the cluster has the effect of slowing down the recession of the Local Group from the cluster by approximately ten percent.

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