The Secret Life of Supermassive Black Holes

If you like astronomy, you have probably heard of black holes—the beasts of the universe. Better stay very far away from them because they devour everything near them in one bite, do they not? Black holes have the reputation of being very hungry objects. In reality, rather than evil beasts, astronomers consider black holes to be the unsung heroes of the galaxies—heroes that eat in a messy way. Black holes are very mysterious, but one thing is sure: without them, our universe would not look the same!

Take the supermassive black holes in the centers of galaxies (Figure 1). As you can tell from the name, they are very, very massive—they can grow to be billions of times the mass of the Sun! There are many interesting and puzzling questions about these odd heroes of the galaxies. For example, how can such powerful objects exist in the first place?

Nowadays, scientists think that supermassive black holes formed in the early days of the Universe, when many smaller black holes merged. The bigger the black holes became, the bigger their appetites. However, it is not yet fully clear the exact roles that supermassive black holes play in the formation and structure of galaxies—but it does not seem a mere coincidence that they usually lie in a galaxy’s center (rather than on the edge), and that the more massive the black hole is, the faster the stars move around it. Over the last few decades, astronomers have accumulated a fair amount of knowledge that can shed light on some of the mysteries of black holes’ messy eating behavior [1].

When Black Holes Throw Up

Photos of supermassive black holes often show a red “doughnut”—a ring of light (Figure 2) [2]. What is this doughnut made of? To explain this, we must look at a black hole’s diet. Massive things have a huge gravitational force, which means they attract other, less massive objects—just as the Earth pulls you back down when you jump. For this reason, supermassive black holes attract and eat up gases, stars, and smaller black holes.

However, they do not swallow everything around them. Gas particles and clouds that do not get close enough to the black hole are not captured, but they rotate around the supermassive black hole because of the black hole’s gravity—like the Earth rotates around the Sun because the Sun is more massive than the Earth. As a result, instead of falling inside the black hole directly, these gas particles run around the black hole, like they are on a merry-go-round, and they serve as the black hole’s food supply. It is actually a bit messier than a standard merry-go-round, because the gas particles hit each other while rotating around the black hole [3]. Imagine a merry-go-round combined with a bumper car ride!

The closer the gas particles get to the black hole, the more energy they lose in the forms of heat and light. The result is a super bright disk structure that surrounds the black hole and looks like a doughnut. Astronomers call this an accretion disk—accretion means growing by accumulation. Supermassive black holes are the largest objects in the galaxies, and their accretion disks can be trillions of times brighter than the Sun. In fact, astronomers saw the radiation from these “doughnuts” as far back as 100 years ago, long before we could take pictures of supermassive black holes themselves!

At this point, you might be thinking, “That is so cool!” However, supermassive black holes, gas particles, and gravity are only some of the players in this messy drama. There is one additional character that we have not yet mentioned: magnetic fields. Interstellar gas particles with positive or negative charges carry weak magnetic fields. However, when gas particles get really hot in accretion disks, they can form strong magnetic fields that influence the movement of new gas particles that approach the disk or the black hole. These approaching gas particles experience a tug-of-war between the black hole’s gravity and the disk’s magnetic field, pulling them in opposite directions—while gravity pulls the gas particles toward the black hole, the magnetic field forces them outwards in a spiral path. If the magnetic force is stronger than the gravitational force, the gas particles fly away from the black hole, in a direction perpendicular to the disk, and start to form their own structure. Imagine a candle sticking out of the “doughnut”. Through a powerful telescope, it looks like the black hole has thrown up its food. We call these outward flows jets because they can travel very quickly, often reaching close to the speed of the light. Jets can be powerful and shine very brightly. If the black hole is really big, its jets can travel the distance of several thousand to millions of light years away from the black hole, so these jets have a big impact on the rest of the galaxy [4]. But do not worry, we are safe here on Earth: the closest big black hole to us, Sagittarius A^*, does not have very energetic jets.

Jets never fail to surprise us: they are usually launched out of a black hole in pairs, but some black holes have only one jet. Some have jets that are bent in the middle, and some have several jets that are thrown out at different times—as if these black holes take breaks from time to time [5]! Sometimes jets are very dim; other times they are very powerful. There are still so many things to discover, but astronomers have been working quite hard on jets—by doing fancy calculations on supercomputers, and by building big radio telescopes that can help us look even further and deeper into the sky.

As jets travel in space and pass through gas clouds, they inevitably slow down and interact with the surroundings. Similar to how planes leave white trails behind them in the sky, jets leave interesting structures like filaments and bubbles. The difference is that the trails of jets are not easily detectable with our own eyes, so we need radio and X-ray telescopes to see them (Figure 3). Using these telescopes, astronomers can now take pictures of black holes and their jets: these pictures are not only beautiful, but also help astronomers to figure out the fate and history of the galaxies! Remember—in astronomy, cool pictures can help us understand how the universe works!

The Mystery of the Missing Stars

Like our Milky Way galaxy, many galaxies contain billions of stars and are still actively forming new stars. For example, the Milky Way forms around seven stars per year [6]. Astronomers know that certain types of gases naturally become cradles of star formation when they reach a certain cold temperature. However, according to this theory, we should count a greater number of stars in many galaxies than we actually see. What’s going on? Supermassive black holes are claiming their territory—their super-hot, energetic jets are effectively blowing away and heating up the cold gas clouds in their paths. Therefore, not enough gas is left to form baby stars. Maybe black holes do not like baby stars much!

These actions have consequences, though. As the cold gas is swept away by a black hole’s jets, less gas falls closer to the black hole, which effectively puts the black hole on a diet. The less a black hole eats, the less powerful its jets are. Eventually, the jets would not be powerful enough to fully block the formation of new stars. This often continues like a cycle: as soon as there are enough gas particles around, the black hole quits its diet and starts to attract the gas again. The gas that does not fall into the black hole is blasted by the jets, disturbing the formation of new stars. When too much gas is swept away, the cycle starts again [7].

By counting the star formation rate in galaxies and measuring the temperatures of the gases in the galaxies’ centers, we can figure out how powerful jets are and how efficient they are at sweeping gases away. However, we do not fully understand many details of this process because we are still not completely sure how black holes work. For example, in very rare cases, astronomers have also seen the complete opposite of what we have described here: that is, black holes that spark (rather than hinder) the formation of baby stars [8].

Summary

To sum up, supermassive black holes have very exciting lives and they play a very important role in the formation and evolution of galaxies. The formation of accretion disks and jets, the interaction of jets with gas clouds, and the birth of stars are all pieces of a big puzzle that has captivated astronomers for many decades. We are only starting to piece everything together, and astronomers are still building new telescopes to take more photos of black holes and galaxies, and understand them in more detail.

Glossary

Black Hole: ↑ A place in space where gravity is so strong that even light cannot get out. A supermassive black hole has a mass over 100, 000 times the mass of the Sun.

Galaxy: ↑ A huge collection of gas, dust, stars, planets, and space objects held together by gravity. Our Earth and Sun belong to the Milky Way galaxy.

Gravitational Force: ↑ A force that causes objects to attract each other. Gravity keeps planets orbiting the sun, and the moon orbiting the Earth. It also makes objects fall inside black holes.

Accretion Disk: ↑ A disk of gas and space dust surrounding a black hole, due to gravity. Accretion means growing by accumulation. In photos, it looks like a doughnut-shaped ring of bright red light.

Magnetic Field: ↑ A region around a magnetic material or a moving electric charge where other magnetic objects feel the attraction or repulsion.

Interstellar: ↑ Between the stars in a galaxy.

Jet: ↑ Bright, energetic streams of particles that are launched out of a black hole almost at the speed of light.

Light Year: ↑ The distance covered by light in 1 year, which corresponds to 9.46 trillion kilometers.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.