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A new window on the Universe: The formation and evolution of galaxy clusters and proto-clusters

Final Report Summary - NEWCLUSTERS (A new window on the Universe: The formation and evolution of galaxy clusters and proto-clusters)

LOFAR is an innovative radio telescope that is opening up the low-frequency sky for sensitive astronomical observations. With 38 antenna stations in the Netherlands and 13 antenna stations in 7 different countries in Europe, LOFAR is a truly pan-European telescope. LOFAR does not contain moving parts and its software tools alone ensure that the telescope can point at desired directions. One of the main aims of LOFAR is to produce radio images of large areas of the Northern sky at unprecedented quality. However, making such maps was a major challenge and one of the key goals of this ERC project was to overcome the challenges. A state-of-the-art software pipeline was developed that removed man-made interfering signals, dealt with the corrupting influence of the ionosphere – a layer within the earth atmosphere that makes all ‘the radio stars twinkle’ - and produced huge radio maps taking the non-uniform sensitivity of the individual LOFAR stations into account. As tens of Petabytes needed to be dealt with, a system was designed so that large computers centres could be used effectively. As a result, the largest high-quality radio survey ever was produced, which not only showed the scientific potential of studying the low-frequency radio sky is, but also how fascinating and how beautiful it is. The range of different science topics that these surveys are enabling is huge: galaxies, massive black holes at the centres of galaxies, clusters of galaxies, supernova remnants in our galaxies, exo-planets and cosmology. Already the LOFAR team of 200 members has published more than one hundred papers and many more are in the making. The first big data release in February 2019 attracted a lot of attention from the press. To quote the response of Chris Evans / Captain America (@ChrisEvans | 13,760,210 followers): ‘I can’t even describe how much I love this.’

The second goal of this ERC project was to use these surveys to study clusters of galaxies: These are the most massive objects in the universe consisting of large ensembles of galaxies and contain a copious amount of hot gas and dark matter. Clusters of galaxies grow during collisions with other clusters. These collisions are the most energetic events in the Universe and create enormous gaseous shocks sometimes with sizes a hundred times the size of our own Galaxy. These shocks and associated turbulence can accelerate electrons to relativistic speeds and amplify magnetic fields within the cluster gas. And in the presence of this magnetic field, such relativistic electrons emit low-frequency radio emission. An important focus of the ERC project was to make detailed LOFAR images of colliding clusters in our local Universe to constrain the nature of the mergers. Tens of high-quality maps have been made, constraining the physics of particle acceleration and the nature of magnetic fields in these clusters. Clusters of galaxies became to have properties similar to local clusters at an epoch when the Universe was half of its current age. An important question was whether clusters at this epoch already had developed magnetic fields. LOFAR survey observations showed that about half of such distant clusters contained magnetic fields at levels similar to the local clusters. This discovery already ruled out several models that explain the origin and evolution of magnetic fields in clusters. The progenitors of distant massive clusters are located in the early universe. Very distant powerful radio galaxies are known to be hosted by forming massive galaxies and are an excellent starting point to locate such progenitors. The modelling we carried out in the context of the ERC project showed that the LOFAR surveys should contain about a hundred very luminous objects residing in distant protoclusters. The discovery of the most distant of such objects known was one of the highlights of the ERC project.