Why do galaxies that live in the enormous structures known as galaxy  clusters look different from normal, isolated galaxies, such as our  Milky Way? To answer this question, an international research team led  by MPA has created the Hydrangea simulations, a suite of 24  high-resolution cosmological hydrodynamic simulations of galaxy  clusters. Containing over 20,000 cluster galaxies in unprecedented  detail and accuracy, these simulations provide astrophysicists with a  powerful tool to understand how galaxies have formed and evolved in one  of the most extreme environments of our Universe.

  Galaxy  clusters are giant associations of up to several thousand galaxies,  embedded in diffuse hot gas and invisible dark matter (see Fig. 1).  Observations have shown that these extreme environments influence the  properties of the galaxies within: while isolated galaxies often contain  star-forming discs where massive young stars shine in blue, cluster  galaxies are mostly yellow or red - indicating that they stopped their  star formation several billion years ago. Often, these cluster galaxies  present an apparently featureless “elliptical” morphology. Understanding  the origin of these differences has been a major unsolved problem in  astrophysics for decades.

Fig  1.The Galaxy cluster “Abell 1689”, located approximately 2 billion  light years away, is one of the most massive clusters in the known  Universe. This picture is a composite of an optical image, taken with  the Hubble Space Telescope, and an X-ray observation with the Chandra  Space Telescope. The former shows starlight from more than 1000  galaxies, the latter (in purple) the hundred-million-degree hot gas  which permeates the space between galaxies and contributes more mass to  the cluster than all its galaxies together.