The Milky Way may have two supermassive black holes
Universe

The Milky Way may have two supermassive black holes

Measurements of stars around our galaxy’s core indicate that our 4-million-solar-mass black hole, Sagittarius A*, may be accompanied by another supermassive companion.

Do supermassive black holes have any companions? The nature of galaxy creation suggests that the answer is yes, and pairs of supermassive black holes should be frequent throughout the universe.

As an astrophysicist, I am interested in a wide range of theoretical astrophysics topics, from the origin of the first galaxies to the gravitational interactions of black holes, stars, and even planets. Black holes are fascinating phenomena, and supermassive black holes and their dense star environs represent one of the most extreme regions in our universe.

Sgr A, the supermassive black hole at the center of our galaxy, has a mass around 4 million times that of our Sun. A black hole is a region of space in which gravity is so powerful that neither particles nor light can escape. A tight cluster of stars surrounds Sgr A. Astronomers were able to confirm the presence of this supermassive black hole and determine its mass thanks to precise observations of the orbits of these stars. Scientists have been tracking the motions of these stars around the supermassive black hole for almost 20 years. Based on what we’ve seen, my colleagues and I show that if there is a friend there, it might be a second black hole nearby that is at least 100,000 times the mass of the Sun.

milkywaycore
At the center of our galaxy is a supermassive black hole in the region known as Sagittarius A. It has a mass of about 4 million times that of our Sun. ESA–C. Carreau

Almost every galaxy, including our own, has a supermassive black hole with a mass millions to billions of times that of the Sun. Astronomers are currently investigating why supermassive black holes are frequently found in the center of galaxies. One prevalent theory connects supermassive holes to the potential that they have friends.

To understand this concept, we must return to the time when the universe was only around 100 million years old, to the era of the very first galaxies. They were significantly smaller than today’s galaxies, around 10,000 times or less massive than the Milky Way. The first stars that died in these early galaxies formed black holes that were tens to thousands of times the mass of the Sun. These black holes fell to the galaxy’s core, the center of gravity. Because galaxies evolve by merging and colliding, collisions between galaxies will result in supermassive black hole pairs — the important aspect of this scenario. The black holes collide and grow in size as a result. That is a black hole.

If the supermassive black hole indeed have a companion rotating around it in close orbit, the galaxy’s core is locked in a complex dance. The gravitational forces of the partners will also exert their own pull on the neighboring stars, disrupting their orbits. The two supermassive black holes are orbiting each other, each exerting its own gravitational pull on the stars around it.

The gravitational forces of the black holes pull on these stars and cause them to modify their orbit; in other words, after one circle around the supermassive black hole pair, a star will not return exactly to where it started.

Astronomers can forecast what will happen to stars by using our understanding of the gravitational interaction between the possible supermassive black hole pair and the surrounding stars. Astrophysicists like my colleagues and me may compare our predictions to data, determining possible star orbits and determining whether the supermassive black hole has a companion exerting gravitational effect.

Using S0-2, a well-studied star that orbits the supermassive black hole at the center of the galaxy every 16 years, we can already rule out the existence of a second supermassive black hole with a mass greater than 100,000 times the mass of the Sun and a distance greater than about 200 times the distance between the Sun and the Earth. If there existed such a companion, my colleagues and I would have observed its impacts on SO-2’s orbit.

But that doesn’t rule out the possibility of a smaller companion black hole lurking nearby. Such an item may not affect SO-2’s orbit in a way that we can easily measure.

The physics of supermassive black holes

Recently, there has been a lot of interest in supermassive black holes. The recent sighting of such a giant at the heart of the galaxy M87, in particular, has offered a fresh window into understanding the mechanics of black holes.

blackholeimage
The first image of a black hole. This is the supermassive black hole at the center of the galaxy M87. Event Horizon Telescope Collaboration, CC BY-SA

The proximity of the Milky Way’s galactic core – only 24,000 light-years away – provides a one-of-a-kind laboratory for studying the fundamental physics of supermassive black holes. Astrophysicists, for example, like as myself, would like to learn more about their impact on galaxies’ center regions, as well as their involvement in galaxy formation and evolution. The discovery of two supermassive black holes at the galactic center would imply that the Milky Way fused with another, presumably tiny, galaxy at some point in the past.

Monitoring the stars around us can tell us a lot more. Scientists were able to conduct a one-of-a-kind test of Einstein’s general theory of relativity thanks to measurements of the star S0-2. S0-2 flew past the supermassive black hole in May 2018 at a distance of only approximately 130 times the Earth’s distance from the Sun. According to Einstein’s theory, the wavelength of light emitted by the star should stretch as it moves up from the supermassive black hole’s deep gravitational well.

The stretching wavelength anticipated by Einstein, which causes the star to look redder, was detected, proving that the theory of general relativity accurately describes the physics in this extreme gravitational zone. I’m looking forward to S0-second 2’s closest approach in around 16 years because astrophysicists like me will be able to verify more of Einstein’s predictions regarding general relativity, such as the shift in the direction of the stars’ elongated orbit. However, if the supermassive black hole has a companion, the projected outcome may be altered.

milkywayhubble
This NASA/ESA Hubble Space Telescope image show’s the result of a galactic collision between two good-sized galaxies. This new jumble of stars is slowly evolving to become a giant elliptical galaxy.
ESA/Hubble & NASA, Acknowledgement: Judy Schmidt

Finally, if two huge black holes orbit one other at the galactic core, as my team believes is probable, gravitational waves will be produced. The LIGO-Virgo observatories have been detecting gravitational wave radiation from merging stellar-mass black holes and neutron stars since 2015. These ground-breaking discoveries have provided scientists with a new way to perceive the universe.

Any waves released by our hypothetical black hole pair will be at low frequencies, too low to be detected by the LIGO-Virgo detectors. However, a planned space-based detector called LISA may be able to detect these waves, allowing astrophysicists to determine whether our galactic center black hole is alone or has a partner.

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