Intermediate black hole found in galactic tatters T he Hubble Space Telescope has pinpointed the blue star cluster remains of ashredded galaxy around amedium-sized black hole, dubbed HLX-1 (Hyper-Luminous X-ray source 1). Weighing inat 20,000 solar masses and located in a galaxy 290 million light years away, HLX-1 is providing a lead into understanding the evolution of supermassive black holes. Black holes have been found aplenty with masses comparable to stars, having formed in supernovae, and with masses millions of times that of the Sun atthe centre of galaxies like theMilky Way. HLX-1 isbelieved to be amissing link between the two extremes weighing in at around 100 to 100,000 solar masses. To get a handle on the the nature of this intermediate mass black hole, Sean Farrell of the Sydney Institute for Astronomy and Leicester University and his team simultaneously studied HLX-1 in ultraviolet, visible, infrared and X-rays using Hubble and NASA's Swift satellite, publishing their results in the AsU'ophysical Journal. "We don't know how supermassive black holes form, but it's possible that they grow from lower mass black holes that merge together and steadily gain mass," says Farrell. "If this is correct, then there should be a class of intermediate mass black holes. Up until recently, evidence for their existence was lacking, but the discovery ofHLX-1 has changed that and so now it appears as though the merger scenario for supermassive black hole formation isviable." The large distance to HLX-1 means that Hubble is unable to resolve the individual stars, but the team were able to deduce information from the emitted light, which has an excess of red that cannot be explained by the existence of a gaseous accretion disc alone. Instead, a hot, young cluster of stars must besurrounding HLX-1, but what does this mean for our intermediate black hole? "We believe [due to the existence ofa very young cluster ofstars] that the intermediate mass black hole HLX-1 is very likely to be the remnant of a low mass galaxy that was accreted bythe larger host galaxy ESO 243- 49 less than 200 million years ago," concludes Farrell. "When the dwarf galaxy passed through the disc it would have interacted with this material resulting inthe stripping of stars and gas from the dwarf galaxy, in a sense 'shredding it'.The force of the interaction would have compressed the gas around the black hole, triggering star formation." Gemma Lavender ------_ -----_ ------------_ -----_ --------_ ------_ -----_ --------_ -----_