Astronomers using the Low-Frequency Array (LOFAR) telescope network have detected a vast ridge of radio emission connecting the merging galaxy clusters Abell 0399 and Abell 0401. The results imply that intergalactic magnetic fields connect Abell 0399 and Abell 0401 and challenge theories of particle acceleration in the intergalactic medium.
LOFAR image centered on the Abell 0399-Abell 0401 system; color and contours show the radio emission at 140 MHz; the faceting of the field is shown with red lines. Image credit: Govoni et al, doi: 10.1126/science.aat7500.
Galaxy clusters contain dozens or hundreds of galaxies of all ages, shapes and sizes, vast quantities of hot gas, and large amounts of dark matter.
Typically, they have a mass of about one million billion times the mass of the Sun.
At one point in time they were believed to be the largest structures in the Universe — until they were usurped in the 1980s by the discovery of superclusters, which typically contain dozens of galaxy clusters and groups and span hundreds of millions of light-years.
However, clusters do have one thing to cling on to; superclusters are not held together by gravity, so galaxy clusters still retain the title of the biggest structures in the Universe bound by gravity.
Spanning the space between galaxy clusters are filaments, which form a vast cosmic web.
These clusters form at the nexus of filaments and grow by accumulating cosmic objects that merge into them, which can generate magnetic fields and relativistic particles in the gas.
Dr. Federica Govoni from Italy’s Istituto Nazionale di Astrofisica-Osservatorio Astronomico di Cagliari and colleagues used LOFAR to observe a ridge of radio-emitting plasma extending between Abell 0399 and Abell 0401, two galaxy clusters that are approaching a merger.
This indicates the presence of a magnetic field and relativistic electrons in the filament of the cosmic web that connects the two clusters.
The structure is larger than can be explained by standard models, suggesting a yet unknown particle re-acceleration mechanism operating in the intergalactic gas.
“We performed simulations to show that the observations could be explained by re-acceleration of existing relativistic electrons via shock waves generated in the merger,” the astronomers said.
Their results were published in the journal Science.
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F. Govoni et al. 2019. A radio ridge connecting two galaxy clusters in a filament of the cosmic web. Science 364 (6444): 981-984; doi: 10.1126/science.aat7500
