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MOSCOW, January 14, Vladislav Strekopytov. Researchers from the US and UK have proposed a model to explain strange space objects. Spheres glowing in the radio range were noticed several years ago, but how they appeared was not understood. According to the authors of the new theory, these are shock waves from a powerful explosion in the center of galaxies.
Strange radio circles
In September 2019, the Australian ASKAP (Australian Square Kilometer Array Pathfinder) radio telescope complex, collecting information for the Evolutionary Map of the Universe (EMU), identified three strange ring objects. Clearly visible in the radio wave range, they did not appear in any way in X-ray, optical or infrared. ASKAP recorded the fourth radio circle in 2021.
All these objects resemble bubbles. The edges are brighter, the center is darker. The size is about one minute of arc, which is about three percent of the Moon in the night sky. But since the distance could not be determined, the real scale remained unclear.
Typically, circles in radio astronomy images are associated with planetary nebulae, supernova remnants, circumstellar shells, protoplanetary disks, and spherical star-forming galaxies. Or it could be the result of instrument calibration errors.
But in this case, the radio spectral index of the circles did not correspond to the planetary nebulae, and for supernova remnants they were located too close to each other in the small area of the sky surveyed by the EMU project. Australian scientists decided that they were dealing with a new class of astronomical objects. And they gave it a name: Odd Radio Circles (ORCs), that is, “strange radio circles.”
For a detailed study of one of them – ORC 1 – the world's most powerful radio telescope, MeerKAT of the South African Astronomical Observatory, was used. We obtained higher-quality images, which show the complex structure of the object, and also compiled maps of its internal parameters – polarization and spectral index (dependence of the radiation flux on frequency).
The fifth strange radio circle (ORCJ0102-2450) was found in the archive of data collected by the giant radio telescope GMRT (Giant MetreWave Radio Telescope) in 2013, several years before the inclusion of ASKAP. Visually, it turned out to be much larger than the previous ones, with a less clear outline. At its center, astronomers discovered the elliptical radio galaxy DES J010224.33-245039.5. Having determined the distance to it, they calculated the approximate diameter of the radio circle – about 980 thousand light years.
Later it was established that in the center of two of the four other ORCs there are also radio galaxies, characterized by an increased background of radio emission.
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Russian physicists Alexander Kirillov and Elena Savelova from Bauman Moscow State Technical University suggested that strange radio circles are wormholes, tunnels in the fabric of space-time.
According to another hypothesis, ORC is a shell of synchrotron radiation accompanying a powerful explosion in the center of the galaxy – host, resulting from the merger of two supermassive black holes. The shock wave accelerated electrons in the intergalactic medium, which led to the appearance of a spherical radio bubble.
The supermassive black hole at the center of the galaxy could also eject relativistic jets of high-energy charged particles from its poles. When the plasma cooled in intergalactic space, it formed two diffuse lobes of radio emission, and radio circles are their end sections.
Another possible option: a burst of star formation in the galaxy. Even a galaxy as modest in its rate of new star formation as the Milky Way generates stellar wind from charged particles and gas. During the flare, the kinetic energy of the ejected matter turns into thermal energy, and the rapidly expanding superheated gas spreads in the intergalactic medium.
Be that as it may, experts agree that the emergence of ORC was preceded by some powerful events within the parent radio galaxy, accompanied by ejection of matter and giant shock waves. Judging by the size of the circles, the events that gave rise to them occurred quite a long time ago – about a billion years ago.
Impact echoes of galaxies
Scientists led by Alison Coyle, professor of astronomy and astrophysics at the University of California, San Diego conducted computer modeling, the results of which confirmed one of the hypotheses. Giant spherical clumps glowing in radio rays turned out to be shock waves from starbursts.
These flares are very powerful, the kind that produce a huge number of short-lived massive stars that turn into supernovae. Researchers note that this happens, for example, when two large galaxies collide and merge.
“As a result, all the galactic gas is concentrated in a very small region, which causes an explosion of star formation. Massive stars quickly burn, expelling their gas in the form of outgoing winds “, write the authors of the article.
The gas is pushed out and creates a powerful spherical flow. When the front of its shock wave crashes into the surrounding rarefied intergalactic medium, a radio glow appears. In principle, it is similar to a radio signal from planetary nebulae or supernova remnants, only much larger.
“You need a high rate of mass outflow, and the surrounding gas outside the galaxy must be rarefied, otherwise the shock wave will stop. These are two key factors,” – Coyle is quoted in the press release.
Observations from ORC 4 at the Keck Observatory in Hawaii were used as the primary data for the modeling. It turned out that in the mother galaxy of this radio circle there is an abundance of heated and compressed gas. Another interesting fact is that many stars in this galaxy are the same age – six billion years. That is, at that moment there was an outbreak of star formation.
It ended, according to estimates, about a billion years ago. Before this, for about 200 million years, galactic gas, spurred by supernova explosions, was thrown out at a speed of 450 kilometers per second. Then the galactic wind died down, but the bubble continued to expand, and the reverse shock wave from the collision with intergalactic matter sent the cooler gas back. The presented model covers a period of 750 million years. That's about the same age as ORC 4.
“Strange radio circles have given us a unique opportunity to observe outgoing galactic winds,” Coyle notes. “Now we will determine how common these extreme events are and what their life cycle is.”
Scientists hope answer the question whether the ORC phase is characteristic of most galaxies at a certain stage of their development or whether it requires special conditions. Ultimately, this will expand the general understanding of the evolution of the Universe.
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