Abell 2666 - Galaxy Cluster
The galaxy cluster "Abell 2666" is located at a distance of 340 million light years in the northern part of the constellation Pegasus and is best observed in
autumn. The redshift is z=0.027 and the escape velocity is 7,800 kilometers per second. The apparent size of the cluster is 50 arcminutes, which corresponds
to a physical diameter of 4.9 million light-years at this distance.
Of particular interest is the brightest galaxy of the cluster (NGC 7768) which belongs to the group of "central dominant" (cD) galaxies and was discovered
already in 1828 thanks to its apparent brightness of 14.2mag. The galaxy is slightly oval with a Hubble Type E2 and has a diameter of nearly 180,000 light-years,
which is quite a lot for an elliptical galaxy. Such huge ellipses are thought to be formed by the repeated merging of galaxies. The absolute magnitude of NGC
7768 is M= -22.9mag. With a luminosity of 150 billion suns it is about 5x more luminous than our Milky Way.
In the center of NGC 7768 there is a disk of dust with an apparent size of about 1" (arcsecond), which corresponds to a true diameter of 1,700 light-years.
A supermassive black hole (SMBH) with a mass of 1.3 billion suns has been detected within this disk. Its event horizon would extend to the orbit of
Neptune if it were at the position of our Sun.
The halo of this massive galaxy is populated by a multitude of globular clusters, which are of course very difficult to detect. About 110 candidates have
been found with the Hubble Space Telescope, with magnitudes ranging from 24.5mag to 27mag.
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In my 20 inch telescope "Abell 2666" is relatively easy to observe. When looking through the eyepiece, the first thing to notice is of course NGC 7768,
which is a brighter, slightly oval galaxy with a stellar nucleus.
The other galaxies are in a row, extending in north-south direction. Some of the observed galaxies appear oval to elongated and are easily visible with
indirect vision at a magnification of V=270x.
