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.
Observations of nearby galaxies and high-redshift quasars suggest that black holes are present in the majority of galaxies. The first quasars harbor already black holes as massive as several billion solar masses.
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.