Flying black hole leaves behind a trail of new stars
A chance discovery by the Hubble Space Telescope provides the first evidence of a special black hole. It was ejected from the centre of its galaxy.
New images from the Hubble Space Telescope show a strange structure: a strikingly straight, narrow strip of newly formed stars located far outside a galaxy. The most likely explanation for this is probably an escaped black hole. This is what a team led by Pieter van Dokkum from Yale University writes in a paper accepted by the scientific journal "Astrophysical Journal Letters", which is already available on the preprint server "arXiv".
In the centre of almost every galaxy there is an extremely massive black hole with a mass millions to billions of times that of our sun. It has been hypothesised for decades that these objects could be ejected during collisions between galaxies and move through the universe detached from their home galaxies. However, there has been no evidence of this until now because such isolated black holes are very difficult to detect. The chance discovery by Pieter van Dokkum's team could now show the trail of just such an outlier. The researchers estimate that the extremely massive black hole was ejected from the centre of its home galaxy 39 million years ago and is now racing through the universe at 1600 kilometres per second.
Source: © Van Dokkum, P. et al.: A candidate runaway supermassive black hole identified by shocks and star formation in its wake. arXiv:2302.04888, 2023, fig. 1 (Ausschnitt)
The origin of the streak of stars and gas points towards the centre of a small irregular galaxy. Its shape suggests that it has only recently merged with another world island. The galaxy and the streak are about 5.4 billion light years away from Earth. The strip is over 200,000 light years long - a good twice as large as our Milky Way system. At its tip, which points away from the galaxy, it is brighter and narrower, while it becomes slightly wider near the galaxy. The scientists' analyses show that the stars in the streak are only a few million years old. The black hole at the tip of the streak therefore did not tear them out of the galaxy and "take them with it", but they formed from the gas that was compressed by the black hole's flyby. This interpretation is also supported by the fact that the stars at the tip of the streak appear to be younger than the stars further back.
Although the current images cannot directly depict the extremely massive black hole itself, the team's observations and model calculations clearly speak in favour of its existence. New high-resolution images taken with the James Webb Telescope could show more details in the strip and, above all, better characterise the region around the black hole at the top. According to the scientists, observations of X-rays could even provide direct evidence of the hot swirl of matter around the black hole, the so-called accretion disc.
Spectrum of Science
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Cover photo: © draco-zlat / Getty Images / iStock (detail) The illustration shows two interacting black holes, one of which is being flung away. Astronomers have now found traces of such a process with the Hubble telescope.
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