Scientists have discovered huge and unexplained structures in space that extend across nearly a billion light years and emit a faint radio glow, according to a new study.
These “megahalos” are made of energetic particles, but it is not clear what is supercharging the immense objects or how they emerge within groups of galaxies known as clusters. Answering those questions would not only reveal the origin of the megahalos, it could also shed light on some of the fundamental mysteries of the cosmos, including details about the cosmic web, a network of large-scale structures made of matter and gas that connects the universe.
Scientists led by Virginia Cuciti, an astrophysicist at the University of Hamburg, detected four megahalos while scanning hundreds of distant galaxy clusters with the LOw Frequency ARray (LOFAR), a radio telescope with stations that span Europe that is extremely sensitive at low frequencies.
The four objects are associated with smaller radio halos, which are diffuse structures made of electrons traveling at near-light (relativistic) speeds that have previously been spotted in many galaxy clusters. However, the megahalos are 30 times larger and 20 times dimmer than the normal haloes.
The researchers said that “the existence of megahalos demonstrates that beyond the edge of radio halos, mechanisms operate that maintain a sea of relativistic electrons,” which are guided by magnetic fields, and added that “the mechanisms responsible for the formation of the large-scale emission are still unknown,” according to a study published on Wednesday in Nature.
“The fact that the entire volume of galaxy clusters should be filled with relativistic particles and magnetic fields has been predicted by numerical simulations, so we knew that there must be radio emission at some level even at those large scales,” Cuciti said in an email to Motherboard. “However, such emission was also expected to be faint and therefore, not surprisingly, it has never been detected until new generation sensitive telescopes such as LOFAR came online.”
“While we were analyzing the data of one of the clusters presented in this study, we saw some significant hints of radio emission on very large scales, so we decided to re-inspect all the images of a sample of 310 clusters that we were studying with the aim of looking for similar emission,” she continued. “When we discovered that three other clusters of this sample showed emission on similar scales and with similar characteristics, it became clear that we discovered a new type of cosmic phenomenon that opens the possibility to explore the external region of galaxy clusters through radio observations.”
Galaxy clusters can contain hundreds or thousands of individual galaxies, all gravitationally interacting with each other, making them one of the most radiant and chaotic environments in the universe. While the brightest parts of clusters are often clearly visible to telescopes on Earth, even across billions of light years, less is known about the darker “intracluster” spaces that exist between galaxies in these groups.
Scientists have spotted radio halos at the center of many galaxy clusters, especially those that are merging together, suggesting that electrons are energized by the mergers, and then sculpted into puffy halo structures by strong magnetic fields. The newly discovered megahalos have unique properties that hint at a different energizing mechanism, perhaps related to turbulence that arises due to the dissipation of gravitational energy in the intracluster medium.
“There is still a lot to be understood about megahalos,” Cuciti said. “More sensitive radio observations are needed in order to unveil the properties of these exotic radio sources and understand whether megahalos are present only in some or in the majority, if not all, galaxy clusters. In this respect, we are confident that the upgrade that LOFAR is undergoing (LOFAR 2.0) will allow us to reach the sensitivity needed to answer this question.”
“Thanks to these observations, we now have some measurable information about the external regions of galaxy clusters,” she continued. “As it often happens in astrophysics, simulations will now come to the aid. We plan an extensive campaign of cosmological simulations aimed at reproducing the presence and properties of megahalos.”
These efforts could yield new insights about the cosmic web, which Cuciti described as “an intricate architecture of filaments” that spans the universe. Galaxy clusters form at intersections of these filaments, which are made of gas and an enigmatic substance called dark matter. The clusters grow by accumulating matter that flows from filaments through their darker outer regions, which remain poorly observed. Megahalos have now provided a new means to probe the murky fringes of the clusters.
“The fact that we can probe these regions with megahalos, means that we can have information on how energy is dissipated during the formation of cosmological structure on the large scale and how particles get accelerated in plasmas with very low density,” Cuciti said. “Also, megahalos are an important step forward towards the direct detection of the large scale structure of our universe.”
“The presence of overdensities in the universe, such as filaments, walls, and groups is usually inferred from the observation of galaxies living in these regions, but here we are moving towards the observation of the plasma that entirely fills this space,” she added. “The final aim is to understand the unique conditions of matter in the overdense regions of the universe, track their structure and derive their evolution patterns.”
LOFAR is part of a new generation of ultra-sensitive radio arrays that will revolutionize our view of the universe by exposing exotic objects and mapping the enormous cosmic web, among countless other discoveries. Scientists are also eagerly awaiting the completion of the most sensitive radio telescope ever built, called the Square Kilometer Array (SKA), which should become operational within the next decade
“This is truly a golden age for radio astronomy,” Cuciti said. “LOFAR opened up a new observational window covering the lowest radio frequencies we can observe from Earth (wavelengths of meters to decameters). The result of this work is only one example of the discoveries that have been made since astronomers started exploring this new territory.”
“On the other hand, the unprecedented international effort of SKA will deliver the most sensitive, most complex radio interferometer (a collection of radio telescopes that work together) ever built by mankind,” she noted. “Among other things, these facilities will cover a wide range of crucial topics in modern astrophysics such as the study of the primordial universe, the formation of the first stars and black holes, the exploration of the still mysterious dark energy, and the understanding of the origin of magnetic fields in space.”