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. Visualization of the most massive galaxy cluster simulated as part of the Hydrangea project. The brightness of the image represents the gas density, while the colour encodes the temperature of the gas (blue: cold, white: hot). The hundred-million-degree hot gas in the central cluster is surrounded by a vast network of filaments stretching out into the surrounding Universe. Over a dozen smaller galaxy groups on the cluster outskirts are visible as yellow knots. The bottom-right inset shows the simulated stars, which are clumped into hundreds of galaxies in the cluster centre; each small point represents a galaxy similar to the Milky Way containing several hundred billion stars each. The three panels on the left-hand side zoom in to one individual galaxy, highlighting the vast dynamic range of the simulation. See: http://www.mpa-garching.mpg.de/399296/hl201701