Science & Technology

New research suggests the Milky Way may be host to multiple ‘wandering’ supermassive black holes

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Astronomers have long since concluded that the active galactic nuclei of most galaxies such as our own Milky Way are comprised of supermassive black holes of masses in excess of millions of times that of the Sun. But now researchers at Yale University have uncovered surprising evidence that Sagittarius A* the supermassive black hole (SMBH) at the centre of the Milky Way, itself 4 million times the mass of the sun, may have several siblings of a similar mass magnitude. Unlike the Milky Way’s central supermassive black hole, however, these counterparts do not hold a centrally fixed position instead wandering through their host galaxies. New state of the art simulations imply that our own galaxy may well play host to as many as twelve of these wandering supermassive black holes.

Previous theories regarding these wandering black holes have suggested that a collision and subsequent merger between neighbouring galaxies may force one or more of those central supermassive black holes from their position, thus creating a population of SMBHs away from the galactic centre. NASA reported the possible detection of one of these wandering black holes in October 2016. The black hole candidate was located on the edge of the lenticular galaxy GJ1417+52 for short located roughly 4.5 billion light-years from Earth. The brightness of the object indicates it is a black hole with a mass 100,000 that of the Sun.

Wide-field optical light image from the Hubble Space Telescope. The black hole and its host galaxy are located within the box in the upper left. The inset on the left contains Hubble’s close-up view of GJ1417+52. Within this inset, the circle shows a point-like source on the northern outskirts of the galaxy that may be associated with XJ1417+52. (NASA, https://www.nasa.gov/mission_pages/chandra/main/index.html)

Not at all uncommon black holes

The new research published in the Astrophysical Journal Letters in April indicates that this wandering black hole may not be an outlier, but that such spacetime objects may not be at all uncommon. The researchers used the state of the art cosmological modeller ROMULUS 25 to create simulations of Milky Way mass-equivalent galaxies, of which a sample of 26 was taken and examined. This led the Yale team to determine that a galaxy of the mass of the Milky Way is likely to contain up to a dozen SMBH with five located within 30,000 light years of the galaxy’s centre regardless of that galaxy’s morphology or recent history. In addition to that effects such as gravitational recoil and three-body interactions not consider in the aforementioned simulations, may well cause an increase in the population of wandering SMBHs. The masses of these wandering SMBHs were found to remain close to their initial mass, with only those that stray close to galactic centre able to access material to accrete. Wandering SMBHs may well be detectable by searching for clusters of densely packed stars that researchers speculate may be bound to them.

 

Simulations conducted using the ROMULUS25 modeller show the velocity and direction of travel of SMBHs through their host galaxies (Tremmel, Governato, et al, 2018)

Of course, you may well be wondering if these wandering supermassive black holes may present some danger to our own solar system. Surely the gravitational effects of such an object lurking near Earth undetected would pose catastrophe? Lead author Michael Tremmel, a postdoctoral fellow at the Yale Center for Astronomy and Astrophysics, lays those concerns to rest in a press release.

It is extremely unlikely that any wandering supermassive black hole will come close enough to our Sun to have any impact on our solar system.” Tremmel said, “We estimate that a close approach of one of these wanderers that is able to affect our solar system should occur every 100 billion years or so, or nearly 10 times the age of the universe.” Thus making the likelihood of such an interaction before our own sun has ceased nuclear fusion beyond extremely unlikely.

What can we learn about space from wandering supermassive black holes?

 

a star cluster gathered around a stellar-mass black hole. Such clusters may enable astronomers to detect wandering SMBHs

This work stands to revolutionise the way we think about supermassive black holes and their formation and the composition of galaxies in general. Although the research should be considered to be in an extremely tentative phase, the potential detection of wandering SMBHs should provide a great opportunity to learn more about the dynamics of galactic collisions and about the nature of proto-galaxies and black holes due to their lack of significant growth.  Although work must be done to detect such objects, in doing so, astronomers stand to learn much more about the formation and lifetime of supermassive black holes.

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