Ultraluminous X-ray Sources, High Redshift QSOs and Active Galaxies
  It is shown that all of the 32 point X-ray sources which lie within about 10' of the centre of nearby galaxies, and which have so far been optically identified are high redshift objects - AGN or QSOs. Thus the surface density of these QSOs p similar or equal to 0.1 per square arc minute. Some of them were originally discovered as X-ray sources and classified as ultraluminous X-ray sources (ULXs), nearly all of which lie near the centers of active galaxies. We demonstrate that this concentration around galactic nuclei is of high statistical significance: the probabiliy that p that they are accidental lies in the range one in a thousand to one in ten thousand, and apparently this excess cannot be accounted for by microlensing. 
G. Burbidge, E.M. Burbidge, H.C. Arp, & W.M. Napier