Black holes tend to come in two sizes: relatively small ones that form after a star collapses, and supermassive black holes that contain the mass of millions or billions of stars. Evidence for black holes that sit somewhere in the middle of that range has been scarce.
Now, a new study has found ripples in an interstellar gas cloud that hint at the existence of an intermediate-mass black hole wandering through our galaxy.
Shunya Takekawa at the National Astronomical Observatory of Japan and his colleagues found streams of gas orbiting an invisible source of gravity about 26,000 light years from the centre of the Milky Way. Using the Atacama Large Millimeter Array (ALMA) telescope in Chile, they confirmed the shape of the cloud, which consists of a central clump of gas circled by wispy streams.
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鈥淲hen I checked the ALMA data for the first time, I was really excited because the observed gas showed obvious orbital motions, which strongly suggest an invisible massive object lurking,鈥� he says.
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These so-called high-velocity compact clouds are usually formed by gas colliding with a supernova, which results in the cloud expanding, or collisions with other interstellar clouds that leave behind a V-shaped pattern. They saw neither of these shapes. Instead, the streams of gas are orbiting the central clump so quickly that there must be something else attracting them.
鈥淭he required density is too high for a star cluster, and there is no evidence for light at that location. Hence, the likely explanation is an intermediate-mass black hole,鈥� says Avi Loeb at Harvard University.
The team suggests that a black hole the size of Jupiter is the culprit. Though evidence of intermediate-mass black holes is rare, there are two other potential black holes of this mass near our galaxy鈥檚 centre.
鈥淪uch black holes are believed to exist at the centres of dwarf galaxies and the central bulge of the Milky Way was likely assembled as a result of many mergers of dwarf galaxies or globular clusters,鈥� says Loeb.
Some of these black holes may have been swallowed by the Milky Way鈥檚 central supermassive black hole, he says, but others have escaped its clutches聽are still orbiting around.
They move relatively quickly compared to the gas surrounding them, so they don鈥檛聽absorb聽much of it and therefore don鈥檛 produce much light. This black hole will likely take about 10,000 years to eat up all the gas in the clump and stream that gave away its position, Takekawa says.
鈥淓ven though the evidence is very suggestive, it is not conclusive,鈥� says Loeb. 鈥淥ne would like to detect a telltale signature of a black hole, such as a jet, broad emission line due to relativistic motions, or non-thermal radiation in the X-ray or radio bands.鈥�
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