by XMM-Newton, the European Space Agency and NASA-operated space telescope, has imaged a giant “Leaf Clover” to reveal its mysterious origins.
The Cloverleaf is an example of an “odd radio circle,” or ORC. These objects are strange bubbles of radio light so massive that they can be thousands of times wider than the Milky Way, encompassing an entire galaxy — sometimes, several.
ORCs were discovered in 2019 with the arrival of the aptly named ORC-1 by the Australian Square Kilometer Array Pathfinder (ASKAP). Since then, radio surveys of the cosmos have become sensitive enough to detect seven more ORCs, one of which is the Cloverleaf, the subject of the XMM-Newton observations.
The power required to create such a structure is enormous, leading astronomers to wonder what events might be violent enough to create ORCs. Thanks to the XMM-Newton observations, researchers think the Cloverleaf creation event was a collision between two groups of galaxies.
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“This is the first time anyone has seen X-ray emission associated with an ORC,” Esra Bulbul, the team leader behind this investigation and an astrophysicist at the Max Planck Institute for Extraterrestrial Physics, said in a statement. “It was the missing key to unlocking the secret of Cloverleaf’s formation.”
How to do an ORC X-ray
Numerous computer simulations have attempted to recreate the births of ORCs and have succeeded in recreating the shapes of these strange formations. However, none of them recreated the intensity of the extensive radio emission that characterizes ORC.
Bulbul realized, however, that ORCs had not been studied in X-ray light before. The scientist who rationalizes such a study could be the missing piece of the puzzle. Together with former Max Plank Institute doc Xiaoyuan Zhang, she set out to scan data from the Extended Roentgen Survey with Imaging Telescope Array (eROSITA) in search of such ORC-related emissions.
The duo obtained data on the X-ray emission of a appearance was linked to the Cloverleaf ORC collected during just 7 minutes of eROSITA observing time. Although this was a small amount of data, it was enough to inspire Bulbul to gather a larger team and get five and a half hours of telescope time with XMM-Newton.
“We got really lucky,” Zhang said in the statement. “We saw some plausible X-ray point sources near the ORC in the eROSITA observations, but not the increased emission we saw with XMM-Newton. It turns out that the eROSITA sources could not be from the Cloverleaf, but it was the -strong. to make us look more closely.”
Clash between galaxies
X-ray emission from the Cloverleaf observed by XMM-Newton shows gas distrubution within a group of galaxies embedded in the ORC. This is the equivalent of a chalk outline at a crime scene.
By looking at how this gas was disturbed, the team could see that galaxies within the Clover were part of two separate groups that pulled together, collided and merged.
The X-ray emission also revealed the temperature of the gas in the region, putting it at about 15 million degrees Fahrenheit (8.3 million degrees Celsius). The more galaxies that are involved in a galaxy cluster like this one, the greater the gravitational influence of the merger and the faster the gas is pulled in. All that, in turn, increases the temperature of the falling gas, which means the temperature. this material can give scientists a clue as to how many galaxies were involved in the merger. You would only need to work backwards.
“That measurement allowed us to conclude that the Cloverleaf ORC is hosted by about a dozen galaxies that are gravitationally bound together, which agrees with what we see in deep visible light images,” Zhang said.
As for the radio emissions from the ORC, the team suggests that these were generated by particles accelerated by shock waves that were ejected from the galaxies as they collided.
One issue with the team’s proposal is that galaxy group mergers are common, but ORCs, on the other hand, are rare. This means that it is still uncertain how this particular group of galaxies, which were collapsing together, formed the Cloverleaf, although similar events did not generate such ORCs.
“Galaxies interact and merge all the time,” Kim Weaver, a NASA project scientist for XMM-Newton at NASA’s Goddard Space Flight Center who was not involved in the study, said in the statement. “But the source of the accelerated particles is not clear.”
She added that another possibility for the powerful radio signal could be the supermassive black holes at the core of all colliding galaxies. These black holes may have gone through feeding events and extremes in the past. Electrons may remain remnants from that old activity and could be re-accelerated by the merger of a group of galaxies producing intense radio emissions.
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The mystery of the Leaves may have been solved, but the findings raise more questions about these massive radio emissions. The team will continue to study this ORC to resolve these questions.
“Compounds are the backbone of structure formation, but there is something special in this radio emitting rocket system,” said Bulbul. “We can’t tell right now what it is, so we need deeper and more data from both the radio and X-ray telescopes.”
The team’s research was published on Tuesday (April 30) in the journal Astronomy and Astrophysics Letters.
