The ‘wrong-way’ relic in Abell 3266.Image: Riseley et al
Scientists have spotted mysterious radio structures in the midst of an immense cluster of galaxies located 800 million light years away, reports a new study. These radio objects, some of which have never been seen before, pose new challenges to our understanding of the universe and offer an unprecedented glimpse into tumultuous regions of the cosmic web, a network of filaments and nodes that connects the universe. As the researchers who observed the sources put it in a companion essay, “They defy existing theories about both the origins of such objects and their characteristics.”
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Stretching across 300 million light years of southern hemisphere sky, the galaxy cluster Abell 3266 is one of the biggest structures of its kind in the local universe. As the site of multiple clusters merging together, it opens a window into the fallout of huge cosmic collisions, which is why astronomers have studied it intensely for years, especially in X-ray and visible light. Now, scientists led by astrophysicist Christopher Riseley, a research fellow at the University of Bologna, have examined new images of Abell 3266 captured by the Australian Square Kilometre Array Pathfinder (ASCAP) and the Australia Telescope Compact Array (ATCA), two of the most sensitive radio observatories on Earth.
The data fills in the “comparative lack of detailed radio observations” of Abell 3266—revealing several radio structures for the first time and providing an “unprecedented” look at several others—according to a study published in the Monthly Notices of the Royal Astronomical Society on Monday. The discoveries also fulfill Riseley’s nearly decade-long quest to spot a radio halo, a type of huge diffuse structure seen at the center of some galaxy clusters, among other exotic radio sources.“Out of thousands of known clusters, only around 100 are known to host some combination of relics and/or haloes, so these are fairly rare and enigmatic objects,” Riseley said in an email. “We need to find more of them to really be confident that we understand the details of how they’re generated.”
The newly observed structures in Abell 3266 include a huge radio halo that is “conclusively detected here for the first time” and contains “an extended central diffuse ‘ridge’ that we are as yet unable to classify,” according to the study. The researchers also examined a so-called “wrong-way relic,” an arc-shaped structure with an odd concave shape and other features that have never been seen in similar objects, as well as a “fossil plasma source” that was created by the powerful blasts of a bygone supermassive black hole that has since faded into darkness. Even as seasoned radio astronomers, the team reacted to the otherworldly objects with “excitement, surprise and not a little confusion,” Riseley said. “There was definitely a lot of ‘what the heck?’ and ‘why does it look like that?’ kind of reaction,” he recalled. “ASKAP is such a game-changing telescope that any time you look at a deep image, you find something new and unusual. That’s part of what makes our job so hard, yet so much fun. I’m an observational astronomer, not a theorist, so the challenge of explaining what we see in the context of what we know (or think we know) from theory is really engaging.”According to the researchers, the radio fossil is very ancient. And while they have a decent lead on where it originates, “Our best physical models simply can’t fit the data,” they wrote. “This reveals gaps in our understanding of how these sources evolve—gaps that we’re working to fill.”
“The fossil represents the end stage in the evolution of a radio galaxy,” Riseley explained. “We know how these things age, so we know how the shape of the spectrum should change. Our data mean that this source is very old—the black hole that once powered the fossil switched off a very very long time ago.” “It’s extremely difficult to understand how sources this old age, because we can only see them for a very brief part of their evolution,” he continued. “I think that the only reason we can really see this one is that it exists in a cluster environment—the dense plasma that it lives in provides some additional energy to keep it visible for a little bit longer.”The new observations also show that the arched relic has a rare concave shape and spectral patterns that are “not trivially explained by current radio relic formation scenarios,” the study said. Though scientists know that relics are brightened by shockwaves that ripple through clusters, the spectrum of the wrong-way relic does not slope off across frequencies, like all other structures of its kind, instead displaying sharp breaks in the light pattern, which “cannot be explained by any of the theorists I’ve talked to,” Riseley said. “This strongly suggests that we’re looking at new and unusual physics with the ‘wrong-way’ relic,” he added, “and so our theorists have got to go back to the drawing board to explain how you can get a spectrum like what we see. It’s an extremely exciting challenge!”
The properties of these bizarre radio sources don’t seem to fit in with our current understanding of the origin and evolution of similar structures. Many of these mysteries may be resolved with future observations from ASKAP and ATCA, as well as with an immense project known as the Square Kilometre Array (SKA), which will become the most sensitive radio observatory on Earth when it is completed sometime this decade. Galaxy clusters reside “at the intersections of the large-scale structure of the Universe’s Cosmic Web,” Riseley and his colleagues said in the study, which is an interconnected network of dark matter filaments of mind-boggling proportions that spans the entire universe. While it will be satisfying just to get a better read on these strange objects, there are also bigger questions that could be constrained by studying the radio universe, such as the unexplained origin of cosmic magnetism.“We observers need to work with theorists closely,” Riseley said. “Our models are only good if they continue to stand up to scrutiny, and where new data challenges that, we need to understand why. The ‘wrong-way’ relic challenges what we thought we knew, so we need to understand what conditions are required to generate something like this object.“Maybe it’s something unique to the environment of this cluster, maybe it’s something specific about the merger geometry, maybe it’s a particularly unusual combination of properties,” he concluded. “We don’t know yet, but we’re working to find out.”