Institut de recherche sur les lois fondamentales de l'Univers

Black holes with masses intermediate between the known stellar- mass and supermassive black holes are of great interest because they may represent a previously unknown class of compact object and an intermediate stage in the formation of supermassive black holes. Such intermediate mass black holes may form in the collapse of thefirst generation of stars or via collisions in the cores of dense stellar clusters. Such objects may be visible as extremely bright X-ray sources in external galaxies. I review recent observational results on intermediate mass black holes and ultraluminous X-ray sources.