An even more unusual picture has now been revealed by a rare merging of galaxy clusters. A massive, low-frequency radio 'bridge' linking the two has been discovered by astronomers, covering a distance of 6.5 million light-years. This is proof that a magnetic field connected them before they began to merge.
Only twice has such a radio bridge between merging galaxy clusters been discovered. But already, it's giving us some crucial hints about how these bridges develop.
The galaxy clusters are located in a group known as Abell 1758, around 3 billion light-years distant. The approaching collision of two enormous cluster pair involves four clusters in all.
The tightly connected pair in the north portion, known as Abell 1758N, has previously migrated together and split, with the cluster cores crossing one another roughly 300–400 million years ago, according to X-ray evidence released last year. They'll ultimately turn back around and reunite. Abell 1758S, the pair of southern clusters, are still making their first approach to one another.
The acceleration of electrons during a merger event is assumed to be the source of the radio halo seen in each of these pairings. These couples are now 6 million light-years apart, but the gap is gradually reducing in preparation for a potential four-way cluster. bonk.
The galaxy clusters Abell 0399 and Abell 0401, which became the first merging galaxy clusters shown to have a low-frequency radio bridge joining them last year, are remarkably comparable to this situation.
Astronomers discovered a unique radio emission at 140 megahertz using the low-frequency radio observatory LOFAR, which has 25,000 antennas spread over 51 sites.
Now, LOFAR has been directed to Abell 1758 by a group of astrophysicists from Leiden Observatory in the Netherlands, headed by Andrea Botteon. They found radio emission between A1758N and A1758S at 144 MHz, similar to the radio bridge between Abell 0399 and Abell 0401.
"In their publication, they said that "we corroborate the existence of a massive bridge of radio emission linking the two systems, which was tentatively described in our prior study. This is a cluster pair's second large-scale radio bridge to be discovered so far. At 144 MHz, the bridge is vividly visible in the LOFAR picture, while it is just just detectable at 53 MHz."
This emission is seen as proof that a strong magnetic field links the two clusters. If this magnetic field serves as a synchrotron (particle accelerator), electrons ought to be propelled along it at relativistic speeds, creating synchrotron radiation that may be seen as a low-frequency radio glow.
Another hypothesis, however, involves Fermi acceleration, which speeds up the electromagnetic emission of electrons interacting with turbulence and cosmic shock waves.
Such turbulence and shock waves might be produced in the dense zone between two pre-merging clusters in the early phases of a merger. And according to the research team's results, this may be particularly true if the clusters were already gravitationally perturbed in some manner, such as if each cluster consisted of two smaller interacting clusters.
Two reasons are presented to support this scenario by Botteon and his colleagues. First off, a report published last year found no evidence of a radio bridge in LOFAR data for a pair of merging lower mass clusters, only one of which had a radio halo.
The group also examined observations of Abell 1758 made with the help of the Chandra X-ray Observatory. Additionally, they discovered a strong correlation between the radio emission at 144 megahertz and the X-ray emission, which is in line with the Fermi acceleration scenario's predictions.
The scientists discovered that synchrotron acceleration could not fully explain the enormous distances travelled by the electrons in the merging of Abell 0399 and Abell 0401. After running models, they discovered that the merger's shock waves reaccelerated high-speed electrons, producing an emission that was consistent with the LOFAR findings.
Therefore, it is plausible that there are other forms of acceleration at work; a magnetic field may span millions of light-years of space between galaxy clusters, but shock waves and turbulence provide that last element that makes the bridge complete.
Only two enormous intra-cluster radio bridges have been discovered so far, according to Botteon and his colleagues.
The turbulence (and shocks) produced in the intra-cluster medium during the early stages of the merger, which raise both the radio and X-ray emission between the clusters, are likely associated to the development of these structures, which are among the most massive objects yet detected in the Universe.
In the larger Universe, a significant number of merging galaxy clusters have been found. It could be possible to learn more about what creates these gigantic structures by looking for more of these enigmatic radio bridges.
The results of the study have been published in the RASC Monthly Notices.
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