Astrophysicists have discovered a giant ridge of plasma emitting radio waves that connects two galaxy clusters 10 million light years apart.
This cosmic bridge provides the first direct evidence of magnetic fields between galaxy clusters, astrophysicists report today in the journal Science.
"Typically, we observe emissions related to this mechanism within individual galaxies, and even in galaxy clusters, but radio emissions connecting clusters has never been observed before," said the study's lead author Federica Govoni from the National Institute for Astrophysics (INAF).
It is an exciting discovery astronomers have been predicting for around 20 years, said Australian astrophysicist Melanie Johnston-Hollitt, who was not involved in the study.
"This has long been a sought-after piece of this puzzle of the universe," she said.
The two clusters, Abell 0399 and Abell 0401, are in the process of beginning to merge.
If you could zoom out far enough, you would see that the universe is not an orderly collection of galaxies, uniformly distributed through space. Rather, galaxies, dust and gas clump together along massive thread-like filaments skirting vast voids.
Where these filaments intersect, we get galaxy clusters - hundreds or thousands of galaxies orbiting around a single gravitational centre. This network is called the cosmic web.
"[The cosmic web] really does look like a spider's web, like a really disorganised spiders-on-drugs kind of web," said Professor Johnston-Hollitt, who is the director of the Murchison Widefield Array (MWA) radio telescope in Western Australia.
Using the Low-Frequency Array (LOFAR) radio telescope in Europe, researchers were able to obtain images showing the presence of a very faint radio source.
By comparing these emissions with simulations the team calculated that a faint magnetic field - about 1 million times weaker than Earth's magnetic field - stretched between the two galaxy clusters.
If there are radio emissions, there must be magnetic fields
While the presence of a filament of matter between these two clusters had been confirmed previously by the Planck satellite, the detection of the radio waves by LOFAR indicated something more was going on.
The emissions come from electrons swirling around the magnetic fields that are sitting in between the two galaxy clusters, Professor Johnston-Hollitt explained.
"There's no way that we can see the radio emission just by having the matter there, it has to be a magnetic field as well.
"Understanding magnetism is one of these crucial but very misunderstood things in astrophysics."
Magnetic fields are ubiquitous in the universe, but we do not know how they evolved, how they are distributed throughout the universe or how they affect moving objects like galaxies or galaxy clusters.
While we have known about magnetic fields within galaxy clusters since the 1980s, until this study we did not know anything about the magnetic field strength between galaxy clusters.
"What this has done is it's given us the first real numbers about the strength of magnetic fields … between two clusters, so that's extending our knowledge of magnetic fields," Professor Johnston-Hollitt said.
"Now our job is to go out and verify that this is the way that the universe works in many other places as well, not that this is a particularly weird thing."
Where LOFAR leads the SKA will follow
While simulations in the past have predicted this phenomenon existed, it is "extraordinarily difficult to find", Professor Johnston-Hollitt said.
Part of the problem has been that the radio emissions we have been trying to detect are much brighter at lower frequencies than they are at higher frequencies, but the telescopes we have been using for the last 40 years have only been looking at the higher frequencies, Professor Johnston-Hollitt said.
To detect radio emissions at lower frequencies you need a large number of antennae spread across a wide area for sensitivity and different spatial scales.
LOFAR uses multiple antennae spread over an area of 1,500 kilometres in Europe.
Its observation of the radio ridge between Abell 0399 and Abell 0401 bodes well for its contemporaries such as the MWA, and successors like the Square Kilometre Array (SKA), Professor Johnston-Hollitt said.
"We think that you should be able to detect the signal across the entire cosmic web at a very low level, we should see this sort of filamentary web-like structure in low-frequency radio emissions."
The low-frequency part of the SKA will comprise 130,000 antennas in the West Australian outback, creating an instrument with five times the sensitivity of LOFAR.
"The SKA should see much, much, much fainter emissions," Professor Johnston-Hollitt said.
"What this means is we should detect this sort of faint, diffuse emission of the cosmic web all over the place with the SKA, not just in places where it's particularly bright."