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Astronomers have discovered enormous circular radio features of unknown origin around some galaxies. Now, new observations of one dubbed the Cloverleaf suggest it was created by clashing groups of galaxies.<\/p>\n
Studying these structures, collectively called ORCs (odd radio circles), in a different kind of light offered scientists a chance to probe everything from supersonic shock waves to black hole behavior.<\/p>\n
\u201cThis is the first time anyone has seen X-ray emission associated with an ORC,\u201d said Esra Bulbul, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led the study. \u201cIt was the missing key to unlock the secret of the Cloverleaf\u2019s formation.\u201d<\/p>\n
A paper describing the results was published in Astronomy and Astrophysics Letters on April 30.<\/p>\n
A Serendipitous Discovery<\/strong> Until 2021, no one knew ORCs existed. Thanks to improved technology, radio surveys became sensitive enough to pick up such faint signals. Over the course of a few years, astronomers discovered eight of these strange structures scattered randomly beyond our galaxy. Each is large enough to envelop an entire galaxy \u2013\u2013 sometimes several.<\/p>\n \u201cThe power needed to produce such an expansive radio emission is very strong,\u201d Bulbul said. \u201cSome simulations can reproduce their shapes but not their intensity. No simulations explain how to create ORCs.\u201d<\/p>\n When Bulbul learned ORCs hadn\u2019t been studied in X-ray light, she and postdoctoral researcher Xiaoyuan Zhang began poring over data from eROSITA (Extended Roentgen Survey with an Imaging Telescope Array), an orbiting German\/Russian X-ray telescope. They noticed some X-ray emission that seemed like it could be from the Cloverleaf, based on less than 7 minutes of observation time.<\/p>\n That gave them a strong enough case to assemble a larger team and secure additional telescope time with XMM-Newton, an ESA (European Space Agency) mission with NASA contributions.<\/p>\n \u201cWe were allotted about five-and-a-half hours, and the data came in late one evening in November,\u201d Bulbul said. \u201cI forwarded it to Xiaoyuan, and he came into my office the next morning and said, \u2018Detection,\u2019 and I just started cheering!\u201d<\/p>\n \u201cWe really got lucky,\u201d Zhang said. \u201cWe saw several plausible X-ray point sources close to the ORC in eROSITA observations, but not the expanded emission we saw with XMM-Newton. It turns out the eROSITA sources couldn\u2019t have been from the Cloverleaf, but it was compelling enough to get us to take a closer look.\u201d<\/p>\n Gallivanting Galaxies<\/strong> The X-ray emission traces the distribution of gas within the group of galaxies like police tape around a crime scene. By seeing how that gas has been disturbed, scientists determined that galaxies embedded in the Cloverleaf are actually members of two separate groups that drew close enough together to merge. The emission\u2019s temperature also hints at the number of galaxies involved.<\/p>\n When galaxies join, their higher combined mass increases their gravity. Surrounding gas begins to fall inward, which heats up the infalling gas. The greater the system\u2019s mass, the hotter the gas becomes.<\/p>\n Based on the emission\u2019s X-ray spectrum, it\u2019s around 15 million degrees Fahrenheit, or between 8 and 9 million degrees Celsius. \u201cThat measurement let us deduce that the Cloverleaf ORC is hosted by around a dozen galaxies that have gravitated together, which agrees with what we see in deep visible light images,\u201d Zhang said.<\/p>\n The team proposes the merger produced shock waves that accelerated particles to create radio emission.<\/p>\n \u201cGalaxies interact and coalesce all the time,\u201d said Kim Weaver, the NASA project scientist for XMM-Newton at NASA\u2019s Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study. \u201cBut the source of the accelerated particles is unclear. One fascinating idea for the powerful radio signal is that the resident supermassive black holes went through episodes of extreme activity in the past, and relic electrons from that ancient activity were reaccelerated by this merging event.\u201d<\/p>\n While galaxy group mergers are common, ORCs are very rare. And it\u2019s still unclear how these interactions can produce such strong radio emissions.<\/p>\n \u201cMergers make up the backbone of structure formation, but there\u2019s something special in this system that rockets the radio emission,\u201d Bulbul said. \u201cWe can\u2019t tell right now what it is, so we need more and deeper data from both radio and X-ray telescopes.\u201d<\/p>\n The team solved the mystery of the nature of the Cloverleaf ORC, but also opened up additional questions. They plan to study the Cloverleaf in more detail to tease out answers.\u00a0<\/p>\n \u201cWe stand to learn a lot from more thorough observations because these interactions take in all kinds of science,\u201d Weaver says. \u201cYou\u2019ve pretty much got everything that we deal with in the cosmos put together in this little package. It\u2019s like a mini universe.\u201d<\/p>\n For more information on ESA\u2019s XMM-Newton mission, visit: <\/p>\n<\/p>\n By Ashley Balzer
<\/strong>NASA\u2019s Goddard Space Flight Center<\/strong>, Greenbelt, Md.<\/strong><\/p>\n