Report: The discovery of black holes at the centre of the Milky Way will have major implications for future research

By Valentina Balbi and Michel Destrade, NUI Galway

Researchers at Columbia University recently announced the discovery of 12 Black Holes in the middle of the Milky Way galaxy. Projections estimate that around 10,000 isolated black holes should actually be located in a six light-years wide region. This discovery brings an end to a two-decade-long search for "a black hole density cusp" and will have major implications for black hole hunting and gravitational wave research.

Black holes are invisible regions in the universe where the gravitational pull is so strong that nothing can escape from there, not even the light. That makes them particularly difficult to detect with classical telescopes.

One trick astrophysicists have come up with is to capture the gravitational waves created when a black hole merges with another black hole. But it is extremely difficult to measure gravitational waves, because they travel billions of light years and reach earth with extremely low intensity. In fact, only five gravitational wave events have been recorded so far. The first confirmed detection took place in 2015, and lead to the awarding of the 2017 Nobel Prize in Physics. It was due to the merger of two black holes into a black hole "binary".

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Professor Charles Hailey from Columbia University and his collaborators used an alternative strategy to detect black holes. They searched for the weak but steady X-Ray emissions resulting from a black hole merging with a smaller low-mass star. They concentrated their efforts in the "neighbourhood" of Sagittarius A*, a super-massive black hole located at the centre of our own Milky Way galaxy (our solar system is located at the edge of the Milky Way galaxy, on one of its outermost spiral arms). Until now, no evidence had emerged to prove the theory that there were thousands of isolated black holes at the centre of our galaxy, surrounding super-massive black holes such as Sagittarius A*.

The NASA’s archival data from the orbiting Chandra X-Ray Observatory revealed twelve X-Ray signatures of black holes – low mass stars binaries close to Sagittarius A*. From the density of these binaries among all possible black hole formations, the researchers were able to estimate that there must be about several hundred black hole – low mass star binaries, and tens of thousands isolated black holes hidden in the area within three light years of Sagittarius A*.

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RTÉ News report on how the osition and brightness of over a billion stars in Milky Way were mapped out for the first time

Putting this figure into context, Matt Redman, director of the Centre for Astronomy at NUI Galway commented: "It’s astonishing to imagine all those black holes packed into such a small volume of space. By way of comparison, a similarly sized volume of space centred on the sun would not even encompass the nearest star, Proxima Centauri." 

This cluster of black holes is the closest and most accessible cluster now known, located "only" 26,000 light years away from us, in a galaxy which is 100,000 light years wide.

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From RTÉ Radio One's Morning Ireland, an interview with Irish scientist Dr Brian O'Reilly who was a member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana who discovered gravitational waves

The study, which appeared in April in the scientific journal Nature, will have a strong impact on gravitational wave research. Scientists are now able to estimate how many black holes sit at the centre of the galaxy. This discovery will allow them to estimate which gravitational waves events can be attributed to black holes as opposed to other binary objects (white dwarfs, neutron stars) and supernovae explosions.

Professor Andy Shearer from the School of Physics at NUI Galway commented that "The  presence  of so many black holes in such a confined volume makes black hole mergers, which produce the gravitational waves, more likely to occur. The centre of our Galaxy is an exciting place whose secrets will be revealed by telescopes such as ESO’s massive European Extremely Large Telescope and ESA's LISA, a space borne gravitational wave detector."

Dr Valentina Balbi is a Marie Curie Fellow with the School of Mathematics at NUI GalwayProfessor Michel Destrade is the Chair of Applied Mathematics at NUI Galway and a former Irish Research Council awardee.


The views expressed here are those of the author and do not represent or reflect the views of RTÉ