NASA
The study used the Giant Rice Wave Radio Telescope in India, and it is precisely because of technological advances in such telescopes that the mystery of the huge voids in the Ophiuchus Galaxy Cluster has been solved.
Image source: Jumi wave radio telescope official website
In the deep sky, about 390 million light-years from Earth, sits a giant, the Ophiuchus Galaxy Cluster. If we take our commonly used X-ray observations, it is the second brightest galaxy cluster discovered by humans so far, second only to the Perseus Cluster 300 million light-years away. In recent years, astronomers have gradually unearthed the secrets behind this bright pearl that have been hidden for hundreds of millions of years.
Recently, astronomers from the United States and Australia and other countries have found that the huge and mysterious gas cavity previously observed in the Ophiuchus galaxy cluster is caused by the explosion of the giant black hole in the center of the cluster a long time ago. That is, the central black hole violently releases energy, "smashing" a huge void around it, about 1.5 million light-years in diameter, which can hold 15 galaxies in a row. The research results were recently published in the journal Astrophysics.
Simone Giasintucci, a researcher at the U.S. Naval Research Institute, first author of the article, likens this void to a "giant radio fossil," which shows its long history.
The fog is thick and the source of energy is explored
The researchers said similar voids had been found in other galaxy clusters before, such as the aforementioned Perseus Cluster and the A Plesiosaurus Cluster. But the voids in the Ophiuchus Cluster are unprecedented. They estimate that the energy needed to create the void could be 6 times the energy needed for the largest known void of its kind. The energy produced by the 1 billion solar rays for 1 billion years is less than one percent of the energy that caused the explosion.
How exactly did the void come about? According to previous research, it should be caused by active galactic nuclei.
Active galactic nuclei refer to a class of extragalactic cores with a strong central nuclear region, active and bright. It is generally believed that the "engine" of the active galactic nucleus is a supermassive black hole. Black holes attract matter, forming a rotating accretion disk around it. The matter in the accretion disk constantly falls into the black hole, and the angular momentum lost by the matter is "spit out" by the black hole in the form of energy, forming a high-speed jet in the vertical direction of the accretion disk.
In previous studies, central black holes have been observed in both Perseus and other galaxy clusters. The jets of black holes are like a dazzling beam of light in the darkness, carrying huge amounts of energy that can heat and "expel" the gas that spreads around it, creating a similar hollow scene.
However, the mystery of the empty source of energy in Ophiuchus is very tortuous.
"Back in 2006, the Ophiuchus Void caught the attention of researchers." Lu Youjun, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, said in an interview with the Science and Technology Daily reporter that another group of researchers at that time, through the X-ray data provided by the Chandra Space Telescope, found that there was a "strange curved edge" in the Ophiuchus galaxy cluster, that is, the edge of the hole. But what is puzzling is that the black hole in the center of the Ophiuchus Cluster is very dim and silent, and there is no sign of erupting. On the other hand, at the time, researchers believed that the energy required to create this void was too great, so they denied the possibility that the black hole explosion would cause the hole to form.
Now, why is the active galactic nucleus powered by black holes judged to be the "culprit"?
"The study used radio band data from which researchers found high-energy relativistic electrons that black hole jets can directly and massively produce." Chen Xuelei, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, told Science and Technology Daily that the 2006 study mainly relied on X-ray data, limited by the band region, can not detect relativistic electrons, can only detect relatively low energy electrons, the source of these electrons includes both the central black hole explosion of the galaxy cluster, but also other astronomical events such as the merger activity of galaxy clusters. Therefore, under the support of the research data of that year, the source of energy formed by the Ophiuchus void is still unclear.
"If the results of this study are true, the balance of the mystery will tilt towards the end of the 'central black hole explosion'." Chen Xuelei said.
Silence and tranquility, waiting for the next round
In Chen Xuelei's view, this may not be the result of a one-time brief explosion of the central black hole of the Ophiuchus Galaxy Cluster, but it is likely that after tens of millions or even hundreds of millions of years of long-term accumulation, it has revealed the huge void seen today.
Unfortunately, the nucleus of the Ophiuchus Cluster has become "lazy", the central black hole has also calmed down, the signal released by the central nuclear region is very weak, and future generations may not be able to appreciate the beauty of the "fireworks" in the central nuclear region of the Ophiuchus Cluster in their lifetime.
What causes the active galactic nucleus to fall silent? Chen Xuelei said that according to the available data, the time of active galactic nucleus activity may last tens to hundreds of millions of years. Galactic nuclei need to absorb the surrounding material through the black hole to generate energy, in turn, the black hole jet will blow away the surrounding material, over time, when all the material around the black hole is dispersed to the distance, the black hole will "starve to death", the active galactic nucleus will also fall silent. After a long period of evolution, the scattered matter is re-attracted by the black hole, and the next cycle is opened.
"It can be seen that the silence after the outbreak of the active galactic nucleus is actually inevitable, but human beings only had the corresponding observation means decades ago, and it is difficult to observe the entire process of the explosion to the cessation of the active galactic nucleus on the time scale of human activity." Chen Xuelei said.
Although the galactic nucleus within the Ophiuchus Cluster is no longer active, the huge void it has created has not yet dissipated, which can help us restore the history of the starry sky hundreds of millions of years ago. And the impact of this ancient eruption is far more profound than the empty appearance we have observed.
"An explosion of an active galactic nucleus can inhibit star formation to some extent." Lu Youjun pointed out that the birth of stars is closely related to the high density of cooling gas in the universe, and the black hole jet will heat the surrounding gas, which hinders the pace of star birth to some extent. This phenomenon is known as the active galactic nucleus feedback effect.
"Family" Tim Ding, more puzzles to solve
"This study of the Ophiuchus Cluster expands the case for the 'big family' of large void phenomena caused by active galaxy nuclei in the galaxy cluster, and provides a help for us to study the core link in the galaxy formation process, that is, the feedback effect of active galaxy nuclei." Lu Yujun said.
Unlike similar events in the past, the researchers observed the existence of a cold core inside the Ophiuchus Cluster, and the energy around the cold core to the core dropped sharply from 10keV (kiloelectron volts) to less than 1keV, and the temperature also fell off a cliff. Cold nuclei, on the other hand, are often destroyed by high-temperature jets when black holes erupt.
In Lu Youjun's view, the Ophiuchus Cluster's active galactic nucleus erupted into a long period of silence, and the gas in the galaxy cluster was injected with little energy, which led to the formation of the cold core. "It can be seen that this study is also beneficial to the study of the internal heating and cooling of the galaxy cluster, and thus to understand the early active galactic nucleus feedback process in the constellation Ophiuchus."
The stars are vast and the exploration is endless.
Lu Youjun said that in response to the void phenomenon of the Ophiuchus galaxy cluster, there are still some problems that need to be studied in depth. For example, to further clarify the specific size of the black hole in the center of the Ophiuchus cluster, it is possible to estimate whether the black hole can produce enough energy to form this huge void. Moreover, even if the energy is sufficient, whether the jets "besieged" by many galaxy clusters can be launched to the hollow position and still retain huge amounts of energy is also needed to be further demonstrated.
The study used radio information collected by the Murchison Wide-Field Array in Australia and the Giant Rice Wave Radio Telescope in India. "With the development of low-frequency radio telescopes and more new observation technologies, I believe that more and more similar phenomena will be observed, and there will be more exotic astronomical landscapes waiting for us to excavate and explore." Chen Xuelei said. Yu Ziyue