We have seen that in the Local Group there are no
ellipticals.
In Virgo -- an irregular, cluster with lots of substructure -- there
are
some ellipticals but mostly spirals. In Coma -- a rich, smooth cluster
-- it is mostly elliptical and S0 galaxies. The
ratio of S:(E+S0) drops as the density of the environment rises.
This is called the morphology-density
relationship.
Here is what it looks like for nearby clusters:
(From Dressler 1980)
The
fraction of
ellipticals does not change much with redshift (i.e., time). But the
fraction of S0s increases with time, suggesting that cluster
environments are effective at transforming spirals into early type
disks.
(From Dressler et al 1997)
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As we shall see when we study cosmology, galaxy clusters are the last objects to form in the Universe. Clusters form hierarchically, from smaller groups falling together under the influence of gravity. As they "collapse", they mix together and become virialized.
Galaxy cluster simulation
Which is a more evolved cluster, Coma or Virgo? Why?