“Our computer simulations demonstrate rather directly that the supermassive black holes in the centers of every galaxy have a substantial effect on the properties of large galaxies, and can in particular explain their color evolution.”

In the hierarchical theory of galaxy formation, elliptical galaxies are believed to form from the merger of smaller spiral galaxies. However, it has been a major puzzle why elliptical galaxies appear to be so old and red, with very little ongoing star formation. In our work, we were able to directly simulate the color transformations induced by star formation and, in particular, by the effects of a central supermassive black hole during major galaxy collisions. As a result, we could explain the observed colors of the population of large, massive elliptical galaxies in the local Universe. This helps to resolve an important challenge for the leading theory of galaxy formation.

In our work, we have developed a novel numerical methodology which for the first time allows a study of the joint evolution of galaxies and supermassive black holes. This has also opened up the possibility to directly predict the lifetimes of quasars in different environments, leading for example to the interesting discovery of a relationship between the lifetime of quasars and their peak luminosities. This in turn has led us and others to arrive at a new and highly successful interpretation of the quasar luminosity function and its relationship to elliptical galaxies.

Our computer simulations demonstrate rather directly that the supermassive black holes in the centers of every galaxy have a substantial effect on the properties of large galaxies, and can in particular explain their color evolution. Also, the feedback exerted by the accreting black holes during galaxy mergers establishes a tight relation between the mass of the black holes and the velocity dispersion of their hosting stellar bulges, as observed. Black holes are hence not only exotic objects that accompany galaxies as curious phenomena, but rather play a crucial role in shaping the properties of the galaxy population as a whole.

Galaxy formation and black hole growth are therefore intimately connected and require joint modelling. This point of view is currently leading to a major revision and refinement of galaxy formation theories.

Both Lars Hernquist and Volker Springel have a long standing interest in numerical simulations of galaxy formation, while Tiziana Di Matteo is an expert on the astrophysics of black holes. When we met as a team, the idea to attempt to simulate the influence of supermassive black holes on merging galaxies immediately thrilled us. We encountered a number of challenges and setbacks working towards this goal, largely stemming from the huge dynamic range posed by the physics of this problem. However, we could resolve these issues with physical subresolution models where needed, allowing us to obtain numerically robust and converged results.

Basic research in astronomy has significant cultural interest, as it satisfies the curiosity of many people to better understand the Universe we live in. From a science-policy perspective, our work can perhaps be seen as a good example for the increasing importance of computational physics. Within astronomy, numerical simulations have established themselves as a third pillar next to the classic fields of theory and observation.