An ultradeep ROSAT image (200 ks PSPC and 1000 ks HRI exposure times) in the direction of the Lockman Hole, where the interstellar absorption has an absolute minimum, provides the most sensitive X-ray observation to date.

Intensive optical follow-up spectroscopy is carried out with the Keck telescopes to reveal the nature of the X-ray faintest objects discovered so far. These observations are relevant to determine the cosmological evolution of galaxies with different stages of activity and to reveal the nature of the enigmatic X-ray background radiation.

More than 70% of the 1-2 keV X-ray background has now been resolved into discrete sources, the majority of which are active galaxies, presumably with a central black hole.

The diagram shows the ROSAT log N - log S distribution (source counts as a function of flux) which clearly reveals a deviation from an Euclidean behaviour, a direct evidence for the strong cosmologic evolution of the X-ray source population.