50,000 solar masses – and that’s just a midsize black hole

Astronomers have been looking for medium-sized black holes for a long time. You’ve probably heard about the giant black holes at the center of galaxies and those that start with the mass of one star as a result of a supernova. But as small black holes, like from a supernova, gradually grown into giants, they must pass through intermediate stages sometime. The only problem is that these midsize black holes are not very easy to find.

The Hubble Space Telescope has now delivered some important evidence that such black holes actually exist. In 2006, the Chandra and XMM-Newton X-ray observatories detected radiation outbursts in the X-ray range, which were designated X-ray source 3XMM J215022.4−055108. Researchers conjectured that they might have been produced when a star was torn apart by a compact, massive object – something like a black hole. What was really interesting, however, was that the X-ray radiation wasn’t coming from the center of our galaxy, giving the researchers hope that they might have found a medium-sized black hole.

But first they had to rule out alternative explanations – like a neutron star cooling down. To do that, they pointed the Hubble Telescope toward the source. What they found wasn’t exactly what they had expected: 3XMM J215022.4−055108 isn’t located in the Milky Way, but instead in a dense star cluster that belongs to the outer regions of a different galaxy. But those findings also point precisely to a medium-sized black hole: such formations are expected in the core of dwarf galaxies. The dense star cluster could be the remnants of a dwarf galaxy that lost most of its stars when it merged with a larger galaxy. The black hole would have to be about 50,000 solar masses, as the astronomers have written in a paper.

This Hubble Space Telescope image identified the location of an intermediate-mass black hole (IMBH), weighing over 50 000 times the mass of our Sun (making it much smaller than the supermassive black holes found in the centres of galaxies). The black hole, named 3XMM J215022.4−055108, is indicated by the white circle. This elusive type of black hole was first identified via a telltale burst of X-rays emitted by hot gas from a star as it was captured and destroyed by the black hole. Hubble was needed to pinpoint the black hole’s location in visible light. Hubble’s deep, high-resolution imaging shows that the black hole resides inside a dense cluster of stars that is far beyond our Milky Way galaxy. The star cluster is in the vicinity of the galaxy at the centre of the image. Much smaller images of distant background galaxies appear sprinkled around the image, including a face-on spiral just above the central foreground galaxy. This photo was taken with Hubble’s Advanced Camera for Surveys.
Hubble discovers Black Holes in unexpected places.
This artist’s impression depicts a star being torn apart by an intermediate-mass black hole (IMBH), surrounded by an accretion disc. This thin, rotating disc of material consists of the leftovers of a star which was ripped apart by the tidal forces of the black hole.

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  • BrandonQMorris
  • Brandon Q. Morris is a physicist and space specialist. He has long been concerned with space issues, both professionally and privately and while he wanted to become an astronaut, he had to stay on Earth for a variety of reasons. He is particularly fascinated by the “what if” and through his books he aims to share compelling hard science fiction stories that could actually happen, and someday may happen. Morris is the author of several best-selling science fiction novels, including The Enceladus Series.

    Brandon is a proud member of the Science Fiction and Fantasy Writers of America and of the Mars Society.