Clusters of galaxies are the largest objects in the Universe that are held together by their own gravity, and provide great tools for studying the processes that govern AGN feedback.
A major area of research in X-ray astronomy is the accretion of matter on to black holes and neutron stars in the context of AGN and galactic X-ray binaries. Accretion is an energetic process and can give high luminosities.
During the epoch of reionization the first stars and black holes in the Universe (re-) ionized the Intergalactic Medium. Studying the epoch of reionization will tell us about the first galaxies.
The intergalactic medium (IGM) and not galaxies contain most of the baryons (e.g. H, He, C, N, O) in the Universe. We use spectroscopic observations of high redshift backgound quasars to study this material.