Supermassive Black Hole at the Center of Our Galaxy May Not Be Alone

Two Supermassive Black Holes

Artist’s conception of two black holes entwined in a gravitational tango. Credit: NASA

Do supermassive black holes have friends? The nature of galaxy formation suggests that the answer is yes, and in fact, pairs of supermassive black holes should be common in the universe.

I am an astrophysicist and am interested in a wide range of theoretical problems in astrophysics, from the formation of the very first galaxies to the gravitational interactions of black holes, stars and even planets. Black holes are intriguing systems, and supermassive black holes and the dense stellar environments that surround them represent one of the most extreme places in our universe.

The supermassive black hole that lurks at the center of our galaxy, called Sgr A*, has a mass of about 4 million times that of our Sun. A black hole is a place in space where gravity is so strong that neither particles or light can escape from it. Surrounding Sgr A* is a dense cluster of stars. Precise measurements of the orbits of these stars allowed astronomers to confirm the existence of this supermassive black hole and to measure its mass. For more than 20 years, scientists have been monitoring the orbits of these stars around the supermassive black hole. 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.

Sagittarius A Center Milky Way

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. Credit: ESA–C. Carreau

Supermassive black holes and their friends

Almost every galaxy, including our Milky Way, has a supermassive black hole at its heart, with masses of millions to billions of times the mass of the Sun. Astronomers are still studying why the heart of galaxies often hosts a supermassive black hole. One popular idea connects to the possibility that supermassive holes have friends.

To understand this idea, we need to go back to when the universe was about 100 million years old, to the era of the very first galaxies. They were much smaller than today’s galaxies, about 10,000 or more times less massive than the Milky Way. Within these early galaxies the very first stars that died created black holes, of about tens to thousand the mass of the Sun. These black holes sank to the center of gravity, the heart of their host galaxy. Since galaxies evolve by merging and colliding with one another, collisions between galaxies will result in supermassive black hole pairs – the key part of this story. The black holes then collide and grow in size as well. A black hole that is more than a million times the mass of our son is considered supermassive.

If indeed the supermassive black hole has a friend revolving around it in close orbit, the center of the galaxy is locked in a complex dance. The partners’ gravitational tugs will also exert its own pull on the nearby stars disturbing their orbits. The two supermassive black holes are orbiting each other, and at the same time, each is exerting its own pull on the stars around it.

The gravitational forces from the black holes pull on these stars and make them change their orbit; in other words, after one revolution around the supermassive black hole pair, a star will not go exactly back to the point at which it began.

Using our understanding of the gravitational interaction between the possible supermassive black hole pair and the surrounding stars, astronomers can predict what will happen to stars. Astrophysicists like my colleagues and me can compare our predictions to observations, and then can determine the possible orbits of stars and figure out whether the supermassive black hole has a companion that is exerting gravitational influence.

Using a well-studied star, called S0-2, which orbits the supermassive black hole that lies at the center of the galaxy every 16 years, we can already rule out the idea that there is a second supermassive black hole with mass above 100,000 times the mass of the Sun and farther than about 200 times the distance between the Sun and the Earth. If there was such a companion, then I and my colleagues would have detected its effects on the orbit of SO-2.

But that doesn’t mean that a smaller companion black hole cannot still hide there. Such an object may not alter the orbit of SO-2 in a way we can easily measure.

The physics of supermassive black holes

Supermassive black holes have gotten a lot of attention lately. In particular, the recent image of such a giant at the center of the galaxy M87 opened a new window to understanding the physics behind black holes.

First Image of a Black Hole

The first image of a black hole. Using the Event Horizon Telescope, scientists obtained an image of the black hole at the center of galaxy M87, outlined by emission from hot gas swirling around it under the influence of strong gravity near its event horizon. Credit: EHT

The proximity of the Milky Way’s galactic center – a mere 24,000 light-years away – provides a unique laboratory for addressing issues in the fundamental physics of supermassive black holes. For example, astrophysicists like myself would like to understand their impact on the central regions of galaxies and their role in galaxy formation and evolution. The detection of a pair of supermassive black holes in the galactic center would indicate that the Milky Way merged with another, possibly small, galaxy at some time in the past.

That’s not all that monitoring the surrounding stars can tell us. Measurements of the star S0-2 allowed scientists to carry out a unique test of Einstein’s general theory of relativity. In May 2018, S0-2 zoomed past the supermassive black hole at a distance of only about 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 climbs from the deep gravitational well of the supermassive black hole.

The stretching wavelength that Einstein predicted – which makes the star appear redder – was detected and proves that the theory of general relativity accurately describes the physics in this extreme gravitational zone. I am eagerly awaiting the second closest approach of S0-2, which will occur in about 16 years, because astrophysicists like myself will be able to test more of Einstein’s predictions about general relativity, including the change of the orientation of the stars’ elongated orbit. But if the supermassive black hole has a partner, this could alter the expected result.

New Hubble Image of NGC 3597

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. Credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt

Finally, if there are two massive black holes orbiting each other at the galactic center, as my team suggests is possible, they will emit gravitational waves. Since 2015, the LIGO-Virgo observatories have been detecting gravitational wave radiation from merging stellar-mass black holes and neutron stars. These groundbreaking detections have opened a new way for scientists to sense the universe.

Any waves emitted by our hypothetical black hole pair will be at low frequencies, too low for the LIGO-Virgo detectors to sense. But a planned space-based detector known as LISA may be able to detect these waves which will help astrophysicists figure out whether our galactic center black hole is alone or has a partner. 

Written by Smadar Naoz, Associate Professor of Physics & Astronomy at the University of California, Los Angeles.

Originally published on The Conversation.The Conversation

11 Comments on "Supermassive Black Hole at the Center of Our Galaxy May Not Be Alone"

  1. “I am an astrophysicist and am interested in a wide range of theoretical problems in astrophysics, from the formation of the very first galaxies to the gravitational interactions of black holes, stars and even planets.”

    Thank you SMADAR NAOZ for categorizing today’s approach to astrophysics as “theoretical problems.” However, the danger is that astrophysicists “believe” their theories, as you demonstrate. The very next sentence after the quote above is “Black holes are intriguing systems, and supermassive black holes and the dense stellar environments that surround them represent one of the most extreme places in our universe.”

    So suddenly we’ve gone from theory to very bold statements of fact. You absolutely BELIEVE in black holes, supermassive black holes, and no doubt big bang, dark matter, etc. And the fact that you believe these things which have absolutely not been scientifically proven basically makes you and everyone else that believes them highly compromised as scientists. Real science is not based on an unshakable believe in a set of theories and then approaching the things that don’t fit into those theories as problems to solve. It’s based on scepticism of any theory, and seeking to disprove proposed theories based on the scientific method, logic and especially laboratory testing.

    Might I suggest you look again at the basis of these theories that you believe and separate proof from interpretation, and see how much proof there actually is, and more importantly, how might you set up an experiment that could disprove these theories. And if you can’t set up experiments to disprove these theories, whatever else you’re doing, it’s not science.

    • Pointing a telescope to the stars is an experiment. Making a prediction on the spectrum of a star close to a mass concentartion and measuring it is a way of trying to disproof general relativity.
      Terminology in sciens is differnt compared to the use of language in the public.
      A scientific hypothesis is what regular people call a ‘theory’, meaning: “I have an idea of what might be right, but I’m not sure about it”.
      A scientific theory is a model that describes the observed behaviour.
      In the article Smadar Naoz describes her hypothesis of a second heavy black hole in the center of the milky way. The suggested experiment to disproof this idea is to watch the orbit of the star S0-2 over th next decades.
      You are right about the dangers of believing in a hypothesis (not theory). An idea may sounds so good that you want it to be right.
      But what we see here is an other problem in science, it is to get publicity. The easiest way for this is to come up with a bold hypothesis. The bolder the fatser the media will jump on it and report about it. Some recent exampls are hypothesis of Planet 9 and the hypotheses of it beeing a small black hole.

      • MaSi, when it comes to the astrophysics community, I think at this point such things as black holes, big bang, and dark fill in the blank have gone way past hypothesis and theory and right into the realm of foundational dogma. That’s all that’s taught in universities (as consensus “fact”), that’s what’s funded, that’s what grabs headlines ad nauseam, and any physicist that would dare truly question this dogma would quickly become a laughing stock. But when a group of people absolutely believe in things they can’t see or directly detect or really understand such as the things listed above, and they ascribe to them universal supernatural powers that explain pretty much all the things we do see, seems at that point those people should be moved into the fields of mythology and religion.

        Oh, and Bill, yes, quite happy with the name and medz, thanks.

  2. >Gadfly Giznot
    Nice name. Hope you’re still taking your medz.

  3. Science is incredible… as in: I find it less credible as time goes on.

  4. Wonderful poem about the fat women living next door, one was fat and schizophrenic the other was invisible and a genius…both wore clogs when dancing unless the other was wearing nothing, in which case was the other wearing the others knickers? only the scientist knew…

  5. With clarity, Smadar Naoz, writes, as everyone else seems to understand that “. . .the LIGO-Virgo observatories have been detecting gravitational wave radiation from merging stellar-mass black holes and neutron stars.”

    If two entangled stars eventually merge, the combined “light” emitters send out EM radiation and particle masses.
    As “black” mass sinks, rather than EM emitters, would not black holes curve space-time enough to cause stellar masses to “vibrate”?
    Did Einstein specify the direction of his gravitational waves?

  6. NotABeybladeLover | December 16, 2019 at 9:05 am | Reply

    Beyblade beyblade let it rip

  7. GayzLovTurdFudge | December 16, 2019 at 3:04 pm | Reply

    I have a relevant question: WHO GIVES TWO SHITZ???
    This Star Trek business has no impact on my life or finances and most importantly; not one of us humans will ever live to travel to these useless discoveries!!!
    We are either alive or dead! It’s a very simple equation…

  8. Mohamud Hassan Abdulrahman | December 17, 2019 at 7:03 am | Reply

    This black hole is the door of upper world sa calĺ angels high way to deliver massages from both sides under world to upper world and under world to upper world. More details about this issue please visit translation of Quran to English there are chapters mentioned about this and other interesting massages concerning the universum and more secrets about the stars ,the sun and of course the moon. Everything about our live, existence of the mighty, the past the present and the future.

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