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M87
is a cD galaxy, technically.
Kormendy
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(1982)
distinguishes these from being merely
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giant
ellipticals.
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Extensive
stellar envelope up to 100 kpc
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Only
in regions of enhanced galaxy density (a
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factor
of 100 denser than the average)
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Multiple
nuclei in 25-50% of cDs (a very rare
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thing)
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Regular
cD clusters are systems that have
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relaxed
into dynamical equilibrium.
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