A rare fusion between clusters of galaxies has just revealed an even rarer face. Astronomers have found a large, low-frequency radio ‘bridge’ between the two, spanning a distance of 6.5 million light-years – evidence of a magnetic field that connects them in the early stages of the merging process.
It is only the second time that such a radio bridge has been identified between merging galaxy clusters. But already it provides some important clues as to how these bridges form.
The galaxies are about 3 billion light-years away, in a group called Abell 1758. All in all, four clusters are involved in the impending smash-up – two massive cluster pairs coming together.
Last year it emerged that X-ray data that the closely related pair in the northern segment, named Abell 1758N, had already moved and separated together, the cluster nuclei fit together about 300 to 400 million years ago. They will eventually turn around to come back. The few clusters in the south, Abell 1758S, meet for the first time.
Both of these pairs have a radio halo, thought to be generated by the acceleration of electrons in a fusion event. And it is these pairs that are separated by a distance of 6 million light-years, a hole that slowly closes, for a final four-way cluster … bone.
This scenario is very similar to the galaxies Clusters Abell 0399 and Abell 0401, which last year became the first fusing galaxy clusters to appear to have a low-frequency radio bridge connecting them.
Using the low-frequency radio telescope LOFAR, which consists of 25,000 antennas over 51 locations, astronomers discovered a different radio emission at 140 megahertz.
Now a team of astrophysicists led by Andrea Botteon of Leiden Observatory in the Netherlands has turned LOFAR to Abell 1758. At 144 megahertz they discovered radio emission stretching between A1758N and A1758S, just like the radio bridge between Abell 0399 and Abell 0401.
(Botteon et al., MNRAS, 2020)
“We acknowledge,” she wrote in her paper, “the presence of a giant bridge of radio emissions connecting the two systems that was only cautiously reported in our previous work. This is the second large-scale radio bridge to date. observed in a cluster pair. The bridge is clearly visible in the LOFAR image at 144 MHz and tentatively detected at 53 MHz. “
This emission is interpreted as evidence of a large magnetic field connecting the two clusters. If this magnetic field acts as a synchrotron (particle accelerator), electrons along it must be accelerated to relativistic velocities, allowing synchrotron radiation detection to be detected as a low frequency radio frequency.
But there is another possible explanation – Fermi acceleration, in which electrons interact with turbulence and astrophysical shock waves, and stimulate the electromagnetic emission.
In the close region between two pre-fusion clusters, such turbulence and shock waves could be generated in the initial phases of a merger. And the team’s findings suggest that this might be especially true if the clusters were already severely disrupted in some way – for example, if each cluster was a few smaller interaction clusters in its own right.
Botteon and his team bring two supporting arguments to this scenario. First, a lower mass pair of fusing clusters, only one of which had a radiohalo, showed no evidence of a radio bridge in LOFAR observations, according to a paper last year.
Chandra observation (left) and LOFAR (right). (Botteon et al., MNRAS, 2020)
Second, the team also looked at observations of Abell 1758 taken with the Chandra X-ray Observatory. And they found that the X-ray emission very closely correlated with the radio emission at 144 megahertz – in line with predictions of the Fermi acceleration scenario.
In the merger of Abell 0399 and Abell 0401, the researchers found that synchrotron acceleration could not only account for the large distances covered by the electrons. They ran simulations, and found that shock waves generated by the fusion re-accelerated fast electrons, resulting in an emission consistent with the LOFAR observations.
So it seems likely that there are multiple types of acceleration when playing – that a magnetic field can take millions of light years across the space between galaxy clusters, but shock waves and turbulence add that special something that completes the bridge.
“Only two giant inter-bridge radio bridges have been discovered so far,” Botteon and his colleagues wrote.
“These are by far one of the most gigantic structures observed in the Universe, and their origin is probably related to the turbulence (and shocks) generated in the intra-cluster medium in the first phase of ‘ the merger, which stimulates both radio and X-ray emission between the clusters. “
There are quite a few fusing galaxy clusters identified there in the wider Universe. Finding them for more of these mysterious radio bridges could help figure out what these enormous structures generate.
The study is published in the Monthly announcements from the Royal Astronomical Society.
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