SP1: The evolution of galaxy groups and clusters
E. Pointecouteau, T.H. Reiprich, et al.
From extensive detailed observations of low-z systems associated with studies of z=1-2 group and cluster properties, a large X-ray observatory such as Athena, combining high sensitivity with excellent spectral and spatial resolution, will bring us a unique view of the evolving hot thermal baryons swarming within collapsing structures.
It will bring us the ability to bridge between the era of galaxy formation and early evolution (i.e., beyond z~2-3; traced by the Ly-alpha forest) and the lower redshift Universe where large-scale structures are assembling and virialising.