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For the first time, astronomers have reconstructed the magnetic field of an entire cluster of galaxies, from its central nucleus to its outer limits.
The record-breaking achievement is the product of the deepest-ever observations of the galaxy cluster Abell 2255, located around a billion light-years away — and those observations are thanks to the European radio telescope LOFAR (Low Frequency Array).
Abell 2255 has long been known for its complexity in radio waves. Its vast, diffuse radio emissions are created by particles called electrons racing at near-light, or relativistic, speeds and interacting with magnetic fields of galaxies in the cluster. Thus, using Abell 2255 as a cosmic laboratory to study the universe in radio waves could lead scientists to a better understanding of how magnetic fields come about and evolve. These observations could also help paint a picture of the dynamics of hot gas in galaxy clusters. This could reveal how the largest structures in the universe are constructed.
As part of the LOFAR Galaxy Cluster Ultra-Deep Field project, this team used 224 hours of radio image collection to discover that the distribution of large-scale magnetic fields throughout Abell 2255 (which itself stretches out for several million light-years) are not randomly distributed. Instead, these magnetic fields seem to be organized by the motion of gas that occurred during the formation of this galactic cluster.
"Obtaining very sensitive images of galaxy clusters at radio wavelengths is crucial to understanding how electrons are accelerated to relativistic speeds and magnetic fields are amplified on large cosmic scales," team leader Andrea Botteon, of the Italian National Institute for Astrophysics (INAF), said in a statement. "The complexity of these studies is due to the elusiveness of the radio signal from electrons moving in very weak magnetic fields. We believe that the mechanism that 'turns on' these gigantic radio emissions is linked to the formation process of galaxy clusters."
Botteon added that for this research he and his colleagues combined the deepest radio observations ever made with an innovative data analysis technique that allowed them to reconstruct the shape of a galaxy cluster's magnetic field for the first time.
"The coherence of the magnetic field lines observed in some regions of the cluster suggests that the morphology of the field is intimately linked to the dynamics of the gas in which it resides, where it can be 'stretched' or 'compressed' by the motions associated with the formation of the cluster itself," Botteon said.
This analysis revealed that, in some regions, the magnetic fields follow very specific directions, stretching radially along extended radio emissions. In contrast to this, in regions dominated by shock waves, magnetic fields are orientated at tangents. This suggests magnetic fields in Abell 2255 are carved out by the same dynamics that allow clusters to accrete gas and grow.
This serves as the first observational evidence that the same mechanisms that allow galaxies to grow and cluster, creating the largest structures in the universe, also shape their magnetic fields.
The team's research has been accepted for publication in the journal Astronomy & Astrophysics and is available as a pre-peer-reviewed paper on the repository site arXiv.
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