Astronomers Discover A Magnetic Ridge 10 Million Light-Years Long Between Two Galaxy Clusters

Composite image of the two galaxy clusters and the ridge between them. Xmm-Newton/Planck satellite/ F. Govoni et al. 2019, /M. Murgia/INAF

An international team of astronomers have discovered an incredible feature between two galaxy clusters. By tracking the emission of intergalactic particles, they have observed electromagnetic fields stretching between two clusters for the first time. The findings are reported in the journal Science.

Galaxy clusters are the largest gravitationally bound objects in the universe. They slowly increase in mass by capturing nearby gas and merging with other clusters. They sit at crucial points of matter distribution in the universe, also known as the cosmic web.

The two clusters, located roughly 330 million light-years from Earth, are called Abell 0399 and Abell 0401. A filament of gas that connects the two clusters contains accelerated electrically charged particles, emitting synchrotron radiation and producing a characteristically diffuse radio signal (which is often called a halo). The galaxy clusters themselves have such halos.

The radio ridge, as the researchers call it, stretches for over 10 million light-years. The team used observations and sophisticated simulations to establish that the magnetic field is in the hundreds of nanogauss, roughly the strength that you would get in a magnetic field therapy kit. However, it’s not the strength that matters here, it’s its intergalactic length.

“The pair of clusters Abell 0399 and Abell 0401 is truly exceptional," lead author Dr Federica Govoni, from the Observatory of Cagliari, said in a statement. "For a long time, our group have known that both clusters have a radio halo. More recently, the Planck satellite has shown that the two systems are connected by a thin filament of matter.”

Composite image of the two galaxy clusters and the ridge between them. Dss e Pan-Starrs1/ Xmm-Newton/Planck satellite/ F. Govoni et al. 2019, /M. Murgia/INAF

“The presence of this filament piqued our curiosity and prompted us to investigate whether the magnetic field could extend beyond the center of the clusters, permeating the filament of matter that connects them. With great satisfaction the image obtained with the LOFAR radio telescope confirmed our intuition, showing what can be defined as a sort of 'aurora' on cosmic scales,” she continued.

LOFAR, or Low-Frequency Array, is a large telescope network made of 51 stations and over 25,000 individual antennae spread across the Netherlands, Germany, Ireland, France, Sweden, and the UK. The individual observatories work as a single continental-sized radio telescope. This high sensitivity was key in detecting the faint halo emitted by the magnetic ridge between the galaxy clusters.

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