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The Mystery Behind the Restless Black Hole

‍On March 12th, 2021, a team of astronomers published a study in The Astrophysical Journal in which they presented conclusive evidence of a moving supermassive black hole.
The Mystery Behind the Restless Black Hole

Image of the supermassive black hole in galaxy M87, captured by the Event Horizon Telescope (EHT).

On March 12th, 2021, a team of astronomers published a study in The Astrophysical Journal in which they presented conclusive evidence of a moving supermassive black hole.

Key Points

  • Astronomers found a moving supermassive black hole in a galaxy J037+2456.
  • As of now, there is no explanation as to what is causing this phenomenon, but there are numerous theories.
  • McGill Professor and astrophysicist Darryl Haggard shares her thoughts on this discovery.
  • This piece of evidence could lead to further studies on supermassive black holes.

Why Should We Learn About Black Holes?

Black holes are among the most abundant and powerful astrophysical bodies in the entire universe. In fact, they have a gravitational pull so strong that not even light can escape from it. They are also essential to the universe, for many reasons: black holes contribute to the formation of many important elements in our universe, such as stars and galaxies, they help us stay in orbit so we can survive, etc. On top of that, understanding them helps us gain knowledge about the evolution of our own universe.

The Discovery

Scientists have long thought that black holes could move, but no clear example of this phenomenon had been properly observed – until now. After five years of research, an elite team of astronomers was finally able to catch a black hole in action.

A supermassive black hole (SMBH for short), about 3 million times the size of our Sun, was found speeding at a velocity of about 177 000km/h in the center of its host galaxy, named galaxy J037+2456. These black holes are the largest type known to exist, and they can be found at the center of most large galaxies, including the Milky Way. This is the first sound evidence of this rare occurrence to be recorded.

Before you start to worry, this galaxy is over 230 million light-years away from Earth. Fortunately for us, we won’t get disrupted by a speeding spatial vacuum anytime soon!

The Study

Initially, Dominic Pesce et al., the astronomers behind the study, surveyed the velocities of the SMBH located at the core of 10 galaxies. Simply put, if the velocity of a black hole and that of its respective galaxy are not the same, it implies that the black hole has been disturbed, which is the case for the SMBH of galaxy J037+2456.

To calculate these velocities, the researchers used “H₂0 megamasers”. Essentially, they studied the water found in the accretion disk of each of the ten black holes —  the spiral structure of cosmic material that circulates inwards toward the black hole. As the water orbits, it clashes with the other materials floating in the disk. The electrons surrounding the atoms of the water molecules get excited due to these collisions, and once they return to their lowest energy state, they emit a maser —  a laser-like radio beam that can be detected by radio telescopes on Earth.

Photo of a megamaser, captured by the Hubble Space Telescope. Credit: ESA/Hubble & Nasa - Judy Schmidt

This led the astronomers to find out that 1 out of the 10 galaxies they studied contained a moving black hole.

The Theories

In a galaxy, it is common for smaller black holes to be moving around. For supermassive black holes to be moving within their host galaxy, however, is an incredibly unusual occurrence. SMBHs tend to be immobile, due to their enormous mass that would require an equally enormous amount of force to move them. Hence, scientists are confused as to what exactly is propelling it. There are several theories provided in the study, but the team of researchers is keen on three possibilities:

1. The galaxy J0237+2456 is in the process of merging with another galaxy.

The motion could be the result of an ongoing galaxy merger. As they collide, the objects of each galactic system are disrupted as well, including their central black holes. Since the SMBH from one galaxy is not initially in equilibrium with the star system of the other galaxy, it is possible that the new black hole received a “kick” from the gravitational waves emitted during the process, potentially explaining the SMBH’s motion.

2. The SMBH of galaxy J0237+2456 is the result of the merging of two black holes.

When two SMBHs merge, the force of the collision enables the resulting black hole to recoil, a recoil that can reach a very high velocity (thousands of km s⁻¹) and puts the SMBH in motion.

3. The SMBH of galaxy J0237+2456 is part of a binary system.

In a binary black hole system, there are two black holes in close orbit with one another. This orbital motion could be the motion that is observed. According to Pesce, it is difficult to observe a clear example of binary SMBHs, therefore this could be a case in which researchers identified one SMBH in the system and could have missed the other, possibly due to its lack of maser emissions.

Dr. Haggard points out that, although these are three different theories, they are not mutually exclusive: a galaxy merger can naturally end up with a supermassive black hole binary system. Since the two black holes will also try to come together during a galaxy merger, that is, they will oscillate around each other for a while before fusing into one, this creates a temporary binary system. In this case, we see that the three scenarios interlink with one another.

Simulated image of two merging black holes. Credit: SXS, the Simulating eXtreme Spacetimes project

Although there is no concrete explanation behind the cause of its motion, the discovery of the wandering SMBH in galaxy J037+2456 is nonetheless a step in understanding these gigantic orbiting masses.

Why Is It Hard To Study Black Holes?

In an interview with McGill physics professor/astrophysicist Dr. Daryl Haggard, she explains the difficulty in studying black holes: “They don’t emit any light, it’s one of the main defining characteristics. Light, magnetic fields, and basically all information is trapped inside the event horizon of the black hole. For an astronomer, it means that we never get to see what’s perpetually trapped inside its gravitational sphere of influence. The best we can do is make theories and study as close to the black hole as possible.”

It is difficult to directly study black holes, astronomers generally resort to their surroundings, such as the accretion disks in this case.

Why Is This Discovery Important?

Any research concerning black holes and their system could be crucial in designing experiments on astronomy. For instance, scientists are constantly trying to make new experiments on how two black holes merge and whether black holes emit gravitational radiations during the process. The LISA observatory, for example, a mission set to launch in the 2030s, aims to observe the gravitational waves caused by various astronomical phenomena, notably the merging of SMBHs. Therefore, evidence of a moving SMBH could potentially play a role in fully understanding the merging of black holes.

Lastly, it is generally important for us to study outer space and celestial objects, and it is scientific advancements like these that help us gain knowledge about our universe.

“Humans are inherently curious. We like to know how the whole universe works and what is our place in the universe. People have observed the sky for a long time and are driven to understand it. On a practical level, we develop fascinating and useful technologies from doing astronomical studies, for instance, the first generation of cellphone cameras was originally designed for Astro-imaging. There is so much knowledge and innovation that would not have been possible without astronomy,” explains Haggard.

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