Science & Environment

Astronomers find evidence for unusual class of black holes

An artist's impression of the source HLX-1
Image caption An artist's impression of the source HLX-1 - the light blue object to the top left of the galactic bulge

Researchers say they may have found further evidence for the existence of an unusual type of black hole.

Using the European Southern Observatory's Very Large Telescope (VLT) in Chile, an international team of scientists studied the images of the most extreme ultra-luminous X-ray source, HLX-1.

They say the data about the distance and the brightness of the source shows that it may contain an intermediate-size mass black hole, located some 300 million light years away from Earth.

The results of the study have been published in the Astrophysical Journal.

A black hole is a region of space that has such an extremely powerful gravitational field that it absorbs all the light that passes near it and reflects none.

If confirmed, HLX-1 would be classified as an intermediate-type black hole - something astrophysicists suspected to exist, but for which there have been only tentative detections in the past.

Radiation question

The lead author Klaas Wiersema of the University of Leicester's department of Physics and Astronomy, said that after the earlier discovery of the very bright X-ray source, the astronomers "were very keen to find out just how far away it really is, so that we can work out how much radiation this black hole produces".

"We could see on images taken with big telescopes that a faint optical source was present at the location of the X-ray source, located near the core of a large and bright galaxy," he said.

"We suspected that this faint optical source was directly associated with the X-ray source, but to be sure we had to study the light of this source in detail, using the Very Large Telescope in Chile."

He said that the VLT was able to measure the precise distance to HLX-1 and the data from the telescope allowed the scientists to separate the light of the big, bright galaxy from that of the faint optical source.

"Much to our delight we saw in the resulting measurements exactly what we were hoping for: the characteristic light of hydrogen atoms was detected allowing us to accurately measure the distance to this object.

"This provided conclusive proof that the black hole was indeed located inside the big, bright galaxy, and that HLX-1 is the brightest ultra-luminous X-ray source known."

Galactic centre

HLX-1 is located in another galaxy some 300 million light years from our planet. The study also shows that the source is not a super-massive black hole.

Image caption An artist's impression of a black hole at the centre of a galaxy

Astronomers believe that the centres of most galaxies contain such super-massive black holes, and intermediate black holes might simply turn out to be their progenitors.

"Understanding how super-massive black holes form and grow is thus crucial to our comprehension of the formation and evolution of galaxies, which in turn goes part of the way to answering one of the really big questions: how did our own galaxy form and evolve?" said astronomer Sean Farrell, also of the University of Leicester.

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