Deep and romantic, humans may have witnessed the awakening of a black hole

Produced by: Science Popularization China

Author: Li Zhenzhen (Shanghai Astronomical Observatory, Chinese Academy of Sciences), Shi Xuecao (Shanghai Science and Technology Education Press)

Producer: China Science Expo

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Some time ago, astronomers from the European Southern Observatory and the Millennium Institute of Astrophysics in Chile reported that humans may have observed the "awakening" process of a supermassive black hole at the center of a galaxy for the first time.

This supermassive black hole is located at the center of an elliptical galaxy (SDSS 1335+0728) in the Virgo constellation, 300 million light-years from Earth, and has a mass equivalent to 1 million suns. In the previous decades of observation, it has been in a quiet dormant state. Starting in December 2019, it became extremely active and has continued to this day. The discovery was published in the scientific journal "Astronomy & Astrophysics" under the title "SDSS 1335+0728: The Awakening of a Million-Times Black Hole Mass".

SDSS 1335+0728 Galaxy Core Brightens

(Image source: European Southern Observatory)

What is a black hole?

Black holes are the product of extreme gravity in the universe. Massive objects generally have a force of attraction to the outside world, namely gravity. Massive celestial bodies can bind surrounding objects to their vicinity through their own strong gravity. If an object wants to escape the gravitational bondage of the celestial body, it needs to have a greater speed. For example, if a rocket is launched from the surface of the earth, when the speed of the rocket exceeds 11.2 kilometers per second, it can get rid of the gravitational bondage of the earth and enter space. The stronger the gravitational force exerted by a celestial body on the surrounding area, the greater the speed required to leave it.

If the gravity of a celestial body is strong enough that even light, the fastest moving object in the universe, cannot escape from it, it is called a black hole.

To produce such a strong gravitational effect, the mass of the celestial body must be very large and the radius very small. In our universe, some massive stars will eventually collapse into a black hole after evolution. The mass of this black hole is usually several to dozens of times the mass of the sun. In the center of a galaxy, there is usually a supermassive black hole with a mass of one million to ten billion times that of the sun. This time, the astronomers studied a supermassive black hole at the center of a galaxy.

Black hole concept map

(Photo from: UChicago News)

Since the light in a black hole cannot escape, how do we know it exists? Indeed, a black hole itself cannot be seen directly by us, but the strong gravitational force it exerts on the outside world will expose its existence. By tracking the impact of a black hole on the outside world, people can still capture traces of black holes. For example, one of the scientific achievements that won the 2020 Nobel Prize in Physics was to monitor that many stars in the core region of the Milky Way are revolving around a central position, and from this, it was inferred that there should be a massive black hole there.

The stars in the core region of the Milky Way revolve around a central point

(Image source: Reference 2)

Black hole "sleep" and "awakening"

If a star revolves around such a massive black hole without any other disturbance, its angular momentum will not be lost. The outward centrifugal force generated by the object's rotation resists the inward gravitational force of the black hole, thus achieving a balance and the object will not fall into the black hole. This is the same reason why the moon repeatedly revolves around the earth without falling into the earth. In this case, the black hole and the surrounding matter remain at peace, and the surrounding area of ​​the black hole is in a state of tranquility. From the observer's perspective, the black hole at this time seems to be in a dormant state.

When the matter in the core of a galaxy loses its angular momentum due to some kind of disturbance, the outward centrifugal force decreases and cannot resist the inward gravitational force of the black hole, so the matter will fall into the black hole. During the falling process, these materials will form a disk-like structure. The matter in the disk will rotate rapidly around the black hole while gradually falling into the black hole. This process is called accretion.

When matter falls into a black hole, it releases huge gravitational energy, which is eventually radiated into the universe in various forms of energy. The huge energy radiated by this process can even exceed the total radiation of all the stars in the entire galaxy, making the core of the galaxy very bright. At this time, the black hole is in an active state of swallowing the surrounding matter.

The black hole at the center of the galaxy devours the surrounding external matter and yearns to release energy

(Image credit: Shasthra Snehi)

At present, a large number of black holes have been observed in both dormant and active states, but the switching process between these two states has not been clearly observed.

New discoveries from SDSS 1335+0728

In December 2019, astronomers first discovered that the central region of SDSS 1335+0728 was very bright. At first, they thought it was just an ordinary accretion phenomenon of an active supermassive black hole. However, looking back at the long-term observation data of it over the past 20 years, no signs of its activity were found. In other words, this supermassive black hole has been dormant in the past and has only become active in recent years.

Since 2019, SDSS 1335+0728 has maintained the radiation characteristics of an active black hole, producing strong high-energy X-rays, ultraviolet radiation, and strong infrared radiation. Astronomers have tried to give a reasonable explanation for this unusual phenomenon, and there are currently two more likely explanations.

A star swallowing?

When a star moves too close to a black hole at the center of a galaxy, it will be torn apart by the black hole's powerful gravity, stretched into a long and thin "noodle" structure, and gradually fall into the black hole. This process is usually accompanied by a brief release of energy, causing the originally quiet center of the galaxy to suddenly become brighter. This astronomical phenomenon is usually called a tidal disruption event. It is named so because the principle of this process is similar to the tidal phenomenon of seawater on our earth.

We know that the magnitude of gravity decreases as the distance increases. The distances from the moon to the seawater at different locations on Earth are different. The seawater closer to the moon is subject to greater gravity, while the seawater farther from the moon is subject to less gravity. This difference in gravity causes the entire ocean to deform, causing the seawater to rise and fall, resulting in the tidal phenomenon we see.

Similarly, stars near a black hole experience different gravitational pulls at different locations. The closer a star is to a black hole, the more obvious the gravitational difference becomes. When a star is too close to a black hole, the strong gravitational difference can exceed the star's own binding force, tearing the star apart. The torn star matter gradually falls into the black hole and emits strong radiation. If you observe at this time, you will find that the center of the galaxy becomes prominent and brighter.

The tidal disruption process of a supermassive black hole tearing apart a star

(Image source: HubPages)

In recent years, tidal disruption events have been discovered many times in astronomical observations. So, can the abnormal brightening of the galaxy core this time be explained by such tidal disruption events?

Astronomers have found that it does not conform to the characteristics of a typical tidal disruption event in several aspects. For example, the brightness changes caused by tidal disruption events generally only last for tens of days, at most hundreds of days. However, the core area of ​​the galaxy is still brightening now, more than 4 years after it was first discovered.

Of course, since we don't know enough about tidal disruption events, we can't completely rule out this possibility. Perhaps this is an unusually slow tidal disruption event. For example, what was torn apart by the black hole was not a star, but a smaller satellite galaxy nearby. If this is indeed a tidal disruption event, it would be the longest and faintest tidal disruption event known to humans.

Have you ever been lucky enough to witness the awakening of a black hole?

The characteristics of SDSS 1335+0728 observed this time are not much different from other active galactic nuclei. Matthew Graham, a research professor in the Department of Astronomy at the California Institute of Technology, speculated that perhaps this galaxy is not special at all, and we just happened to capture a special moment.

There is usually a supermassive black hole at the center of a galaxy in the universe. Through observation, it is found that the black holes at the center of some galaxies are in a quiet state, while some are in an active state. Statistical results show that the number of black holes in an active state is far less than that in a quiet state. This means that black holes are active only for a small part of the time.

If the supermassive black hole at the center of the galaxy is compared to a giant beast, then this beast is in a dormant state with no food to eat most of the time, and only in an active stage of hunting and eating for a short period of time. When the food is eaten and digested, it returns to a calm state again.

If the black hole at the center of the galaxy has to go through one or more cycles of transitioning from a quiet state to an active state and then back to a quiet state, why haven't these state transitions been widely observed? This is because the duration of a black hole's state (typically millions of years) is very long compared to the limited history of modern astronomical observations (less than a hundred years). In other words, human observations can only witness a brief moment in the entire black hole evolution process.

Only when this moment happens to correspond to the moment when the black hole just starts to change from a dormant state to an active state, can we have the opportunity to witness the awakening of the black hole. By observing near this time node, our limited observation history can cover both the dormant stage and the active stage of the black hole. Such a critical moment is naturally not easy to encounter, which may be the reason why it has not been seen before.

Conclusion

Currently, astronomers are still paying attention to this supermassive black hole and are preparing to use the James Webb Space Telescope to conduct more in-depth follow-up observations to thoroughly understand the reasons for its abnormal activity. This will help predict whether it will return to calm or continue to be active in the future, and deepen our understanding of the growth and evolution of supermassive black holes at the center of galaxies.

References:

1. Sánchez-Sáez et al. (2024); SDSS1335+0728: The awakening of a ∼106M⊙ black hole

2. Genzel, Eisenhauer & Gilllessen (2010); The Galactic Center massive black hole and nuclear star cluster