Cluster Galaxies

How do galaxies in clusters differ from those in the field? What are the properties of cluster galaxies?

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)

But it was hard to say much about the types of galaxies in distant clusters....

....until Hubble Space Telescope came along.

Here is a picture of MACS J0717.5+3745, a cluster of galaxies at z=0.55 (~2 Gpc away, seen as it was ~ 5 billion years ago). What kind of galaxies do we see?

Cluster galaxies are evolving!

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.

  • What processes could drive this evolution in the galaxies?
  • How would these account for the morphology-density relationship?

(From Dressler et al 1997)

Also, the clusters themselves are evolving!
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?

Galaxy clusters are a great laboratory for studying the evolution of galaxies and the evolution of the universe...