Astronomers find evidence for medium-weight black holes

BBC : Black holes typically come in two sizes, big and small. The big ones, which weigh around a million solar masses, lie at the cores of galaxies. The small ones, which weigh around 10 solar masses and comprise the remnants of collapsed stars, are found outside galactic cores. Intermediate-sized black holes are theoretically plausible, but evidence for their existence has been hard to find. Now, Klaas Wiersema of the University of Leicester in the UK and his colleagues have proven that a certain very bright x-ray source called HLX-1 lives in the outskirts of a distant galaxy called ESO 243-49. The off-center location in ESO 243-49 all but confirms HLX-1 as a black hole of at least 500 solar masses. A stellar black hole can’t produce the observed x-ray flux, while a supermassive black hole can’t live anywhere other than at a galaxy’s center.