1. News update
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Intermediate black hole
found in galactic tatters
T
he Hubble Space Telescope has pinpointed the blue star cluster
remains 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 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 at the centre of galaxies like the Milky Way. HLX-1 is believed to
be a missing 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 is viable."
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 be surrounding HLX-1, but what does this mean for
our intermediate black hole?
"We believe [due to the existence of a very young cluster of stars] that
the intermediate mass black hole HLX-1 is very likely to be the remnant
of 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 dwarf
galaxy passed through the disc it would have interacted with this material
resulting 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 gas
around the black hole, triggering star formation."
Gemma Lavender
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