What makes galaxies come to life?

But in the more detailed investigations of this black hole, everything was not completely clear. From the brightness of the mass of light around the black hole, scientists have estimated the speed of matter falling onto "Sagittarius A" (Sagittarius A), i.e. the central black hole of the Milky Way, and found that it is not fast at all, and this means that there is little material on It spills out.

Real image of the Milky Way's central black hole
Credit: ESO/NSF

Priya Natarajan, a cosmologist at Yale University, compared the galaxy to a broken head, noting: Small droplets can get through. Somehow, only one-thousandth of the matter that makes its way into the Milky Way from the surrounding intergalactic medium is able to reach the central black hole itself, and this represents a big problem. Where does the rest of this gas go and What happens to the gas flow? It is clear that our understanding of the growth of the black hole is ambiguous. And there are their central black holes. "Ramesh Narayan", a theoretical astrophysicist at Harvard University, said about this: "It was not clear how black holes cause the formation and control of the evolution process of galaxies. But now there has been a very big change in this field."

These giant holes, or more precisely, superdense matter that even light cannot escape from their gravity, like the engine of the galaxy> are, but researchers have only just begun to understand how they work.

Gravity pulls dust and gas toward the center of the galaxy, forming a rotating accretion disk around its supermassive black hole. Then due to strong forces, the temperature of these materials increases and they turn into white hot plasma. Then, when the black hole swallows this matter, either gradually or suddenly, in a feedback process, energy is returned to the galaxy.

Elliot Quatart "(Eliot Quataert), a theoretical astrophysicist of Princeton University, referring to the computer simulation of this process, said: "When you grow a black hole, it produces energy and pours it into the surrounding environment with greater efficiency than any other process we know in nature. It is this energy feedback that affects the rate of star formation and the gas flow patterns throughout the galaxy."

But still about this period of intense activity of massive black holes, that is, when the so-called "active galactic nuclei" ( AGN), researchers have only a few vague ideas. What is the mechanism of stimulation and the beginning of this process and What is the key to its end are among the basic questions that according to Kristen Hall, a researcher at the Harvard-Smithsonian Center for Astrophysics, scientists are still trying to find their answers.

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Graphic diagram of a distant young galaxy with an active galactic nucleus (AGN)
Credit: NASA, ESA and J. Olmsted (STScI)

The process of "stellar feedback" which occurs during the supernova explosion of a star, has effects similar to the feedback process of an AGN, on a smaller scale. These stellar engines are powerful enough to influence small dwarf galaxies, but only giant engines or supermassive black holes can dominate the evolution of even the largest elliptical galaxies.

In terms of size, the Milky Way galaxy The Milky Way is a normal, medium-sized spiral galaxy. Despite few obvious signs of activity at its center, our galaxy was long thought to be dominated by stellar feedback rather than AGN feedback.

However, recent observations suggest that AGN feedback> also affects it. So astrophysicists hope that with Studying the details of the interaction between these feedback mechanisms in our galaxy and further investigating mysteries such as the current shutdown of the Arc E* black hole at the center of the Milky Way will lead to an answer to the overall evolution of galaxies and black holes.

According to Natarajan, the Milky Way galaxy is becoming the most powerful astrophysical laboratory for scientific research, and as a small universe, it may hold the key to questions in this field.

Galactic Engines

In the late 1990s, astronomers generally accepted the existence of black holes at the centers of galaxies. By then, they could see close enough to these invisible objects to infer their masses from the motions of nearby stars.

But in these studies, a strange trend was observed. : The more massive the galaxy is, the heavier its central black hole is. A relationship that, according to Tiziana Di Matteo, an astrophysicist at Carnegie Mellon University, was "absolutely definitive and a revolutionary finding." In fact, the black hole is kind of talking to the galaxy.

This correlation is especially surprising when you consider that the black hole, as big as it is, is only a tiny fraction of the entire galaxy. For example, Arc E* has a mass of approximately 4 million times that of the Sun, but the mass of the Milky Way is about 1.5 trillion times that of the Sun. For this reason, the black hole's gravity pulls material towards itself only in the innermost region of the galaxy.

A graphic design of the Milky Way and the location of the Earth and the central black hole; A black hole is only a small fraction of the entire galaxy. Credit: National Science Foundation/Keyi Onyx Li , provided a natural way to explain the connection of relatively small black holes to large galaxies. Two decades before that, in the 1970s, he had correctly hypothesized that the cause of the production of bright jets observed in some distant and bright galaxies, i.e., quasars, is their very massive black holes.

He even suggested with Donald Lynden-Bell that the existence of a black hole could explain the glow of the center of the Milky Way. So could these be signs of a general phenomenon that governs the size of supermassive black holes everywhere?

The idea was that the more matter a black hole swallows, the more active and luminous it becomes, releasing gas with momentum and It emits more energy. Ultimately, this outward pressure prevents the surrounding gas from falling into the black hole.

According to Reiss, "it was argued that such a process would disproportionately stop the growth of the black hole." Or according to Di Matteo, "The black hole first puts the surrounding material in its mouth and then swallows it." A very massive galaxy puts more weight on the central black hole, making it harder to eject gas from the black hole. This, in turn, causes the black hole to swell before it swallows up the material.

However, few astrophysicists were convinced that the energy from collapsing matter could be ejected in such a dramatic way. "When we were working on this for our thesis, we were all obsessed with black holes as the point of no return into which gas escapes," said Natarajan, a Rice student who helped develop the first AGN feedback models. Everyone had to do it carefully and seriously, because it was a very disruptive hypothesis.

Finally, the feedback idea was confirmed a few years later, with the help of computer simulations conducted by Di Matteo and astrophysicists Volker Springel. It was developed by Volker Springel and Lars Hernquist.

We wanted to reproduce the amazing array of galaxies that we see in the real world, said Di Matteo. They were aware of the basic trend: galaxies start out small and dense in the early universe. Then, over time, gravity causes these dwarf galaxies to collide and merge in spectacular fashion, forming them into a variety of ring, spiral, cigar, and other shapes.

Galaxies grow in size and diversity until When they become big and calm after enough collisions with each other. Which, according to Di Matteo, "ends up in a lump." In simulations, he and his colleagues were able to recreate these large blobs without distinct structures, called elliptical galaxies, by merging spiral galaxies multiple times. But there was one problem.

The NGC elliptical galaxy 1316
Credit: ESO

While spiral galaxies like the Milky Way have many young stars that glow blue, galaxies Giant ellipticals contain only very old stars that glow red. "They are red and dead, but every time the research team ran their simulations, it showed elliptical galaxies with stars that glowed in blue light, and nothing that could be seen," Springel of Germany's Max Planck Institute for Astrophysics said of the simulation. The process of star formation was not recorded in the computer model."

He added: "To solve this problem, our idea was to enhance the merger process of our galaxies by creating supermassive black holes in the center of the galaxy. We let these black holes gobble up gas and release energy until, like the pressure in a pressure cooker, everything splits apart. With this change, the elliptical galaxy suddenly stopped star formation and turned red and dead. "We didn't expect the effect to be so strong."

By reproducing elliptical galaxies with red, dead stars in this simulation, theories of black hole feedback proposed by Rees and Natarajan were strengthened. that a black hole, despite its relatively small size compared to the entire galaxy, can influence it through a feedback process.

During the last two decades, computer models suitable for simulating large parts of the universe have been refined and expanded. found and are generally consistent with the types of galaxies we see around us.

These simulations also show that the energy ejected from the black holes fills the intergalactic space with hot gas that, if not for the black hole's energy, would have It was cold and turned into stars. Pointing to this power of black holes, Springel said: "So far, various people have been convinced that massive black holes are very plausible engines for galaxies, and no one has come up with a successful model without considering the presence of a black hole."

Simulating the galactic feedback process
Credit: NASA's Goddard Space Flight Center

The secret to the solution Not Feedback

Despite the advances made, computer simulations are still surprisingly incomplete. As matter crawls inward and into the accretion disk around the black hole, friction reduces energy, and the amount of energy lost along the way is something the coders still put into their simulations by hand and by trial and error. This shows that the details are still fuzzy.

Quartart said: "It is possible that in some cases we get the right answer because of a mistake. "Maybe we're missing what's really important about how black holes grow and how they dump their energy into the environment."

The truth is that astrophysicists still don't know the details of AGN feedback. According to Di Matteo, We know how important the feedback process is, but What exactly causes it is unclear. The key problem is that we don't understand the feedback in a deep, physical way.

They know that some energy is emitted as radiation and causes the centers of active galaxies to glow. Strong magnetic fields also cause matter to be ejected from the accretion disk as scattering galactic winds or as powerful narrow jets.

A mechanism that Black holes are thought to eject jets by the Blandford-Znajek Process identified in the 1970s, but What determines the strength of the jet and What How much of its energy is absorbed by the galaxy is still an unsolved mystery. It is directly related to the galaxy, it is even more mysterious than the previous one. According to Springel, "The billion dollar question is, how does energy interact with this gas?"

A graphical representation of the feedback process in the quasar GB1508+5714
Credit: NASA/CXC/M. WEISS

One thing that suggests there is still a problem is that the black holes in advanced cosmological simulations are smaller than the observed sizes of real massive black holes in some systems.

To stop the trend. Forming stars and creating red, dead galaxies, the simulations require black holes to eject so much energy that they choke off the inflow of matter and stop the black hole growing. Comparing reality and simulations, Natarajan said: "The feedback process in the simulations is very aggressive and stops this growth very quickly."

The Milky Way, however, presents a completely different problem: As predicted by most simulations, a galaxy By this size, it should have a black hole between 3 and 10 times larger than the E* arc. That's why, by looking more closely at our galaxy and nearby galaxies, researchers hope to be able to more precisely understand the AGN feedback function.

Milky Way biome

In December 2020, researchers with the eROSITA X-ray telescope reported that they had observed a pair of bubbles tens of thousands of light-years away. They are stretched above and below the Milky Way. These broad X-ray bubbles were similar to the still puzzling gamma-ray bubbles detected 10 years earlier by the Fermi Gamma-ray Space Telescope.

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Meanwhile, two different theories about the origin of Fermi bubbles are still hotly debated. Some astrophysicists believed they were the remnants of a jet stream ejected from the *A arc millions of years ago. Others thought that the bubbles were the accumulated energy of many stars exploding near the center of the galaxy; That is, some kind of stellar feedback.

Gamma-ray bubbles
Credit: NASA's Goddard Space Flight Center

When Hsiang-Yi Karen Yang, a researcher at National Tsinghua University in Taiwan, saw the image of the X-ray bubbles, he knew that if If both X-rays and gamma-rays were produced by an AGN jet, they could share a common factor, such that the X-rays are emitted by the excited gas in the Milky Way, not the gas jet.

He and Co-authors Ellen Zweibel and Mateusz Ruszkowski developed a new computer model, the results of which were published in Nature Astrophysics in spring 2022. The modeling not only reproduces the shape of the observed bubbles and a bright shock wave front, but also predicts that they formed over 2.6 million years from a jet that has been active for 100,000 years; something that cannot be explained by stellar feedback alone.

Thus active galactic core feedback may be much more important than even in typical disk-like galaxies, including the Milky Way. which was thought before. According to Young, "the picture that is emerging is that of an ecosystem in which AGN feedback and stellar feedback are intertwined with the hot, diffuse gas that surrounds galaxies, the paragalactic medium." In different types of galaxies and at different times, different effects and flow patterns will prevail for this interaction. Reveal processes. For example, Europe's Gaia Space Telescope has mapped the precise positions and motions of millions of stars in the Milky Way, allowing astrophysicists to trace the merger history of the Milky Way with smaller galaxies.

The Gaia satellite has measured the positions and velocities of millions of stars and other objects in the Milky Way.
Credit: S. Payne-Wardenaar/K. Malhan, MPIA

Scientists hypothesize that such merger events shake up matter in supermassive black holes, activating them and causing them to glow suddenly and even eject jets. . "There is a big debate about whether the integration process is effective or not," Quatart said. The Gaia data show that the Milky Way did not undergo mergers when the Fermi bubbles formed, and mergers cannot be considered as the drivers of its AGN jet." encounter and activate it. In this way, the black hole can switch completely randomly between states of absorbing matter, throwing energetic jets, blowing galactic winds, and calming down.> From the central black hole of the Milky Way, which shows falling matter, also raises a new question. Astrophysicists already knew that not all the gas that is drawn into the galaxy will reach the event horizon of the black hole, because the galactic winds exert an outward pressure against this accretion flow.

But the power required by these winds to explain Such a very narrow stream is unrealistic. "I don't see a big wind in the simulation, and it's not the kind of wind we need to fully explain what's going on," Narayan said, referring to this.

Nested simulation

Part of the challenge of understanding how galaxies work is related to the large difference between the length scales of stars and black holes and the scale of entire galaxies and their surroundings. When simulating a physical process on a computer, researchers choose a scale of reality and place relevant effects at that scale. But in galaxies, large and small effects interact.

Narayan noted in this context: "The black hole is really small compared to the big galaxy, and you can't put them all in one. Massive simulation. Each structure needs the information of the opposite structure, but the important thing is the way of communicating between them." Now, in order to fill this gap, researchers are launching a project that uses nested simulations to build A coherent model uses how gas flows in the Milky Way and the active galaxy Messier 87.

In this modeling, according to Narayan, "you let information from the galaxy get to the black hole and tell it What to do, and then You let the black hole's information bounce back and tell the galaxy What to do. It's a loop that repeats itself.

These simulations should help clarify the pattern of gas flows in and around galaxies. In this context, further observations of the paragalactic environment by the "James Webb Space Telescope" will be effective. It is an important part of this whole ecosystem. "How do you get the gas into the black hole in the simulation to produce the ejected energy?"

Most importantly, in the new scheme, all inputs and outputs must be consistent between simulations at different scales. and leave little room for doubt. "If the simulation is set up correctly, it will consistently decide how much gas should reach the black hole," Narayan said. So we can look at it and ask why didn't it eat the whole thing? Why was it so crowded and using up so little gas?

The group hopes to use the new research to create a series of images of galaxies at different stages of their evolution. Currently, much about these galactic ecosystems is still speculative. "It's really a new era, where people are starting to think about these different and similar scenarios," Yang said. I don't have a clear answer, but I hope to be able to answer in a few years."

Cover photo: A graphic of a gyroscope and a galaxy
Credit: Olena Shmahalo, Quanta Magazine