Fast-moving stars around an intermediate-mass black hole in ω Centauri

Black holes have been found over a wide range of masses, from stellar remnants with masses of 5-150 solar masses (M-circle dot), to those found at the centres of galaxies with M > 10(5)M(circle dot). However, only a few debated candidate black holes exist between 150M(circle dot) and 10(5)M(circle dot). Determining the population of these intermediate-mass black holes is an important step towards understanding supermassive black hole formation in the early universe(1,2). Several studies have claimed the detection of a central black hole in omega Centauri, the most massive globular cluster of the Milky Way(3-5). However, these studies have been questioned because of the possible mass contribution of stellar mass black holes, their sensitivity to the cluster centre and the lack of fast-moving stars above the escape velocity(6-9). Here we report the observations of seven fast-moving stars in the central 3 arcsec (0.08 pc) of omega Centauri. The velocities of the fast-moving stars are significantly higher than the expected central escape velocity of the star cluster, so their presence can be explained only by being bound to a massive black hole. From the velocities alone, we can infer a firm lower limit of the black hole mass of about 8,200M(circle dot), making this a good case for an intermediate-mass black hole in the local universe.