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Intermediate black holefound in galactic tatters

The Hubble Space Telescope has pinpointed the blue star clusterremains of a shredded galaxy around a medium-sized black hole,dubbed HLX-1 (Hyper-Luminous X-ray source 1). Weighing

in at 20,000 solar masses and located in a galaxy 290 million light yearsaway, HLX-1 is providing a lead into understanding the evolution ofsupermassive black holes.

Black holes have been found aplenty with masses comparable to stars,having formed in supernovae, and with masses millions of times that ofthe Sun at the centre of galaxies like the Milky Way. HLX-1 is believed tobe a missing link between the two extremes weighing in at around 100 to100,000 solar masses.

To get a handle on the the nature of this intermediate mass blackhole, Sean Farrell of the Sydney Institute for Astronomy and LeicesterUniversity 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 possiblethat they grow from lower mass black holes that merge together andsteadily gain mass," says Farrell. "If this is correct, then there should bea class of intermediate mass black holes. Up until recently, evidence fortheir existence was lacking, but the discovery ofHLX-1 has changed thatand so now it appears as though the merger scenario for supermassiveblack hole formation is viable."

The large distance to HLX-1 means that Hubble is unable to resolvethe individual stars, but the team were able to deduce information fromthe emitted light, which has an excess of red that cannot be explainedby the existence of a gaseous accretion disc alone. Instead, a hot, youngcluster of stars must be surrounding HLX-1, but what does this mean forour intermediate black hole?

"We believe [due to the existence of a very young cluster of stars] thatthe intermediate mass black hole HLX-1 is very likely to be the remnantof a low mass galaxy that was accreted by the larger host galaxy ESO 243-49 less than 200 million years ago," concludes Farrell. "When the dwarfgalaxy passed through the disc it would have interacted with this materialresulting in the stripping of stars and gas from the dwarf galaxy, in a sense'shredding it'. The force of the interaction would have compressed the gasaround the black hole, triggering star formation."

Gemma Lavender

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