eROSITA bubbles: giant structures of hot gas
The eRosita space telescope is a scientific instrument that observes X-rays with an energy of between 0.5 and 10 keV. It was designed by the Max-Planck Institute for Extraterrestrial Physics in Germany and was flown on the Spektr-RG satellite placed in heliocentric orbit in July 2019.
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This telescope scans the entire sky about twice a year and records a large amount of data, enabling astronomers to make some surprising discoveries. Among these, he took pictures in 2020 of two giant hourglass-shaped structures resembling the famous Fermi bubbles discovered a decade earlier in higher energy areas.
When they discovered these two huge bubbles in 2020, astronomers realized that they were much larger than the Fermi bubbles. They estimated that the size of this structure is 50,000 light-years, almost the size of the Milky Way.
Scientists have also determined that these bubbles were probably caused by the injection of enormous amounts of energy from the interior of our galaxy into the galactic halo already mentioned for the Fermi bubbles, whose energy released by their gamma radiation varies from 1 to 100 GeV and whose temperature is estimated at 9700 ° C.
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A powerful eruption of activity from a supermassive black hole
Now, scientists are suggesting that the eROSITA bubbles are the result of the activity of a supermassive black hole in the center of our galaxy. This black hole, which appears to be dormant at the moment, could have been active 2.6 million years ago and “spit out” matter for about 100,000 years.
The research team’s results indicate that the Fermi bubbles were discovered in 2010 and the eROSITA bubbles were probably formed by the same energy beam from the supermassive black hole in the Milky Way.
Researchers put forward two hypotheses to explain Fermi bubbles and eROSITA bubbles. The first hypothesis, which is not maintained but which could bring an explanation, assumes the presence of a star at the end of its life and its implosion in supernovae and the expulsion of matter.
The second hypothesis, maintained by scientists, is the presence of a huge black hole in the heart of our galaxy. This supermassive black hole produces a beam of particles that are projected in the form of bubbles under the influence of a powerful magnetic field. This magnetic field is the result of the rotation of the disk very close to the black hole.
With these new data, astronomers were able to reassess the colossal dimensions of this hourglass-shaped structure. eROSITA bubbles are much larger than Fermi bubbles. Each bulb in this hourglass-shaped structure measures between 11 and 14 kiloparsecs (kpc) high for a total height of 22 to 28 kpc, while the overall dimension of the Fermi bubbles is 18 kpc. In terms of width, the eROSITA bubbles show 14 kpc against 6 kpc for the Fermi bubbles.
These dimensions are simply phenomenal. Knowing that a parsec represents 3.26 light-years, we can calculate that the eROSITA bubbles together measure between 72,000 and 91,000 light-years! To realize the extent of its numbers, our galaxy has a diameter of 30 kpc and our sun is 8 kpc from the center of the galaxy.
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A form of high energy radiation
Astronomers have been interested in these structures because they are located in the heart of our galaxy, making it easier to collect data compared to objects at extreme cosmological distances. This relative proximity makes it possible with much more precision to determine the amount of energy present inside these bubbles and the time it took to produce them.
The only thing scientists are sure of is that in order to have produced such gaseous structures, the supermassive black hole at the heart of our galaxy must at some point have been tens of thousands of times more active than it is today. .
Scientists do not yet know exactly what these eROSITA bubbles contain, especially as they contain the Fermi bubbles, the contents of which are still unknown.
It must most likely be matter consisting of high-energy cosmic rays and highly ionized gases that contain chemical elements. What remains is to discover their abundance, their degree of ionization, their density as well as the temperature of the gas, which in any case must be of the order of several thousand degrees.
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Source: Yang, HY.K., Ruszkowski, M. & Zweibel, EG “Fermi and eROSITA bubbles as remnants of previous activity in the galaxy’s central black hole”, Nature astronomy (2022), https://doi.org/10.1038/s41550-022-01618-x