- Author: Lin Yanxing | Editor-in-chief of EASY Astronomy and Geosciences Station, postgraduate student of the Institute of Astronomy of Qing University, trying to look up at the stars in the gutter
Have you seen the Milky Way?
If you travel to a mountain without light pollution at midnight on a clear summer day, you will see a faint, vaguely bright band in the sky, like a thin cloud across the night sky, which is where we live The galaxy ─ ─ the Milky Way (Milky Way) disk. With the blessing of digital cameras, we can still see that this thin disk is actually full of stars, nebulae, and dust lanes, which are complex, deep and beautiful.
But if you have a pair of eyes that can see “gamma rays”, you will see two huge oval “bubbles” with a viewing angle of 50 degrees high and 40 degrees wide, standing on both sides of the Milky Way disk.They’re called “Fermi Bubbles,” and they’re one of the gigantic and mysterious structures in the Milky Way.
The origin of Fermi bubbles, as well as the meaning of their existence, has been the subject of considerable interest to astronomers over the past decade.
Recently (March 2022), a study published in Nature Astronomy showed that the magnificent Fermi bubble is likely to have originated from an energy burst of the supermassive black hole at the center of the Milky Way more than two million years ago.
Discovery of Fermi bubbles
When we hear the word “Fermi bubble”, the first question that comes to mind is often:
“Who is Fermi? What does this bubble have to do with him?”
In physics, Enrico. The name Enrico Fermi is a household name. He was one of the most important physicists of the early 20th century, participated in the Manhattan Project, designed and built the world‘s first nuclear reactor and atomic bomb; and made outstanding contributions in quantum mechanics, nuclear physics, particle physics and statistical mechanics. There are countless physical concepts and research projects named after him in later generations. Among them, the “Fermi Gamma-ray Space Telescope” is included.
True to its name, Fermi is a dedicated gamma-ray space telescope that launched in 2008 and is one of the best gamma-ray space telescopes in orbit. Compared with its predecessors, Fermi has a larger field of view, higher sensitivity and spatial resolution, and can see wider, darker and clearer.
Its main task is to constantly scan the entire sky, draw an all-sky map of gamma rays, and study the most energetic celestial bodies in the universe such as black holes, neutron stars, and supernovae.
Just two years after the Fermi Space Telescope was launched, astronomers have discovered from observations that if we combine the known stars in Fermi’s all-sky gamma-ray map (such as the diffuse gas of the Milky Way, neutron stars, other galaxies, etc.) ) all subtracted, you will see a pair of huge elliptical regions 50 degrees high and 40 degrees wide on the upper and lower sides of the center of the Milky Way, and this is a new structure of the Milky Way that has never been discovered!
Astronomers named it “Fermi Bubble” in honor of the important contribution of the Fermi Space Telescope.
In addition to the Fermi bubbles seen in gamma rays, astronomers have also seen similar structures in microwave and X-ray wavelengths.
In the microwave band, both the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite (Planck) observed two elliptical bright regions at the location of the Fermi bubble, which astronomers call “microwave thinness”. fog microwave haze”. In the X-ray band, the eROSITA satellite, which was launched in 2019, discovered a larger bubble-like structure similar to Fermi bubbles, called “eROSITA bubbles”.
In addition, in the ultraviolet wavelength band, although it is impossible to directly see the bubble-like structure, astronomers can calculate the expansion rate of the Fermi bubble by using the absorption lines generated when distant celestial bodies pass through the rarefied gas in the Fermi bubble. On the order of hundreds to thousands of kilometers per second.
Based on the above data, astronomers believe that the Fermi bubble should have originated from a huge explosion in the center of the Milky Way galaxy millions to 10 million years ago. The explosion released about 1048 – 1049 The sheer amount of energy in joules (equivalent to the energy released by the sun in its lifetime, multiplied by more than 10,000 times) heats the gas at the center of the galaxy, causing it to expand violently at thousands of kilometers per second. Today, a million years later, it has become a huge bubble spanning tens of thousands of light-years.
However, this intricate puzzle is still missing the most core piece:
Where does such a huge amount of energy come from?
Supernova explosion or black hole jet?The Mystery of the Fermi Bubble
Not long after the Fermi bubble was discovered, astronomers put forward two candidates for the core engine that drives the Fermi bubble:
The first view is that there may have been a large number of stars formed in the center of the Milky Way tens of millions of years ago. Among them, young stars will soon finish their lives due to their short lifespan, and explode in supernovae, releasing huge energy. .
Another view is that the supermassive black hole at the center of the Milky Way may have ingested a large amount of gas in a short period of time millions of years ago, and released energy in the form of jets or outflows in the process.
Both statements sound plausible, and astronomers have seen similar phenomena in other galaxies, so how do you know which is right? At this time, the astronomers were divided. The observers continued to make more observations of the Fermi bubble, looking for more possible hidden clues; the theorists used computer simulations to try to reproduce the observations in the computer. result.
In the early years, the two hypotheses had their own strengths and their own inexplicable weaknesses. But with the accumulation of observational data, astronomers gradually found that the jet hypothesis of black holes seems to be more consistent with the observational results, and therefore more convincing. But even so, it is still a very difficult challenge to reproduce all the observed features of Fermi bubbles at once in computer simulations.
Three wishes, one fulfillment
However, in March 2022, Professor Yang Xiangyi of the Institute of Astronomy, Qing University used three-dimensional magnetohydrodynamics computer simulation (MHD Simulation) to reproduce the three important phenomena of Fermi bubble, Yirosita bubble and microwave mist at one time. observed features.
They hypothesized that the supermassive black hole at the center of the Milky Way had ejected two jets toward the upper and lower sides of the galactic disk 2.6 million years ago. Jet with 1050 Joule’s powerful energy, which contains a large number of high-energy electrons moving at close to the speed of light. When these high-energy electrons collide with low-energy photons, the electrons transfer energy to the photons, like a pin being hit by a bowling ball, turning the photons from low-energy visible light into high-energy gamma rays. This mechanism, called “Inverse Compton Scattering,” allows us to see Fermi bubbles in gamma rays.
At the same time, when these high-energy electrons move in the Milky Way’s magnetic field, they emit microwaves and radio waves in the form of “Synchrotron Radiation”, forming the microwave haze we see. Finally, when powerful jets hit the gas in the Milky Way, they create shock waves that travel at thousands of kilometers per second. Wherever the shock wave goes, the compressed and heated gas emits X-rays, which become the Irosita bubbles we see. And the speed of the gas movement is also consistent with the results of ultraviolet observations.
The results of this research, all the observations of gamma rays, X-rays, ultraviolet rays, and microwaves, are beautifully reproduced with black hole jets, which is undoubtedly a major advance in our understanding of Fermi bubbles.
Future Outlook
So, is the mystery of the Fermi bubble’s life experience concluded?
Hmm…not so fast.
No matter how detailed the simulation is, it is an approximation and simplification of the real world, and theorists can always continue to consider more physical mechanisms and calculate more refined results. Observational astronomers will continue to come up with more and better instruments to challenge the results of the simulations.
From a macro perspective, if the supermassive black hole at the center of the Milky Way was really so active more than two million years ago, did the enormous energy it released have had other effects on the Milky Way? Can we learn more about the history of the Milky Way and the complex co-evolutionary mechanisms between black holes and galaxies? These are all to be continued to explore by astronomers.
The story of the Fermi bubble is not over yet.
Thank you
Thanks to the first author of the paper, Professor Yang Xiangyi of the Institute of Astronomy of Qing University, for his guidance and suggestions on this paper.
References (academic papers)
- Fermi and eROSITA bubbles as relics of the past activity of the Galaxy’s central black hole | Nature Astronomy
- Unveiling the Origin of the Fermi Bubbles – NASA/ADS
- X-Ray and Gamma-Ray Observations of the Fermi Bubbles and NPS/Loop I Structures – NASA/ADS
- Fermi Gamma-ray Space Telescope: High-Energy Results from the First Year
Extended reading (reports and popular science articles)
- Related to this study
- Fermi bubble correlation
- Other related astrophysics popular science articles
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