CLEVeR · The CLuster and group Environment as Viewed by eROSITA

Galaxy groups play a crucial role in modern cosmology by providing the key test for the predictions of models of large-scale structure and galaxy formation and evolution. This is because their hot gas content, and thus their X-ray appearance, is determined by the fundamental ingredient of the modern cosmological models, i.e. the feedback of the supermassive black hole hosted by the central galaxy. However, differently from galaxy clusters, within this general framework, the predictions of the galaxy group baryonic content very vastly depend on the feedback implementation. For this reason, they are the ideal laboratory to provide the ultimate constraint for anchoring the simulations. At present, there are no observational constraints to guide the theory and to discern among the different scenarios. The aim of the present proposal is to fill this fundamental gap in our understanding by characterizing the galaxy group population in terms of baryonic content, from their hot gas on Mpc scale to their galaxy population properties on kpc scale. The main limitation to doing so, thus far, has been the lack of X-ray observations of galaxy groups, due to the low sensitivities of previous surveys. With CLEVeR we will overcome this by taking advantage of the unprecedented statistics and sensitivity at the group mass scale of the eROSITA All Sky Survey in the X-rays. In addition, we will complement eROSITA data with upcoming 4MOST spectroscopic surveys of galaxy groups in the optical and with ancillary datasets that capture specific physical processes of the feedback and the central galaxy itself, such as MaNGA, MUSE, and LOFAR. With such an ultimate dataset, we will, then, be able to study and constrain all relevant components on all relevant scales to nail down the physics of galaxy groups and of their interaction with black hole feedback and, from there, of the large-scale structure simulations of the Universe.