In 2019, astronomers discovered mysterious radio circles hundreds of thousands of light-years across. By comparison, the Milky Way Galaxy is more than 100,000 light-years across. What causes these circles? Finally the mystery seems to have been solved.
Radio circuits are also called “individual radio circuits” or ORCs. Large circles are visible at radio frequencies and are strongly circular. It is not visible in the visible, infrared or X-ray spectrum. In total, astronomers found five such objects.
The circles arise from supernova explosions, a team of scientists reported in a paper published in the journal Nature. When stars explode at the end of their lives, they emit hot gases. Suppose several stars explode in a short time, which can happen in active star-forming regions, and then the gas gains enough speed to escape the galaxy. The speed of this gas can reach 2000 kilometers per second. This is slightly less than 1% of the speed of light (300 thousand kilometers per second).
“These galaxies are very interesting,” says Alison Coyle, research leader. “When two galaxies collide, gas is compressed together. This creates many stars in a short time. These massive stars also die quickly, creating fast galactic winds.”
Coyle and her colleagues solved the mystery by looking at ORC 4, one of the radio circuits found. They found that the stars in ORC 4 were about six billion years old and that the period of active star formation occurred a billion years ago. Researcher Cassandra Luchas conducted a number of computer simulations to simulate the radio circuit, and found that the galactic winds were active for about 200 million years before they stopped.
Not many ORCs have been found yet, but that's because the conditions have to be perfect. A lot of material must be ejected in a short time and the gas outside the galaxy must be of low density, otherwise the shock wave will stop. “These galaxies are rare, but they do exist,” Cowell concludes.
Now that astronomers know what causes ORCs, it's time to answer an important follow-up question. Did all major galaxies have a phase where they had a radio circuit? In this way, scientists learn more about the evolution of galaxies.
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