Afbeelding van de twee clusters.

Cosmic aurora lights up the connection between two galaxy clusters

Afbeelding van de twee clusters.

For the first time ever, a stream of magnetic fields and relativistic electrons along a filament that connects the clusters of galaxies Abell 0399 and Abell 0401 has been identified. Thanks to the data collected by the LOFAR radio telescope, it was possible to discover and measure this phenomenon in radio waves for the first time. The study was published today in Science.

In the Universe, matter is distributed along a so-called "cosmic web" consisting of filamentary structures, at the intersection of which huge concentrations of thousands of galaxies form, known as “clusters”. Clusters of galaxies can be considered as the largest gravitationally-bound structures in the Universe. Up to now, a magnetic field in the filaments connecting the clusters had never been observed. Observations with radio telescopes had previously shown a "halo" of radio emission in the central areas of some clusters, confirming the existence of a magnetic field.

Despite being immense, filaments of the cosmic web are in fact extremely rarefied and difficult to detect. “The unique sensitivity to detect extended emission probed by LOFAR is the key to unveil this 'cosmic aurora' in intergalactic filaments for the first time.” says Emanuela Orrù from ASTRON, the Netherlands Institute for Radio Astronomy.

"Understanding the nature of this radio emission and its impact on the large scales structure evolution in the Universe is an open challenge for astrophysicists. We are very excited that thanks to LOFAR we can infer the presence of magnetic fields with intensities as low as hundreds of nanoGauss over cosmological scales." says Marco Iacobelli from ASTRON.

The international LOFAR telescope consists of a European network of radio antennas, connected by a high-speed fibre optic network spanning seven countries. LOFAR was designed, built and is now operated by ASTRON (Netherlands Institute for Radio Astronomy), with its core located in Exloo in the Netherlands. LOFAR works by combining the signals from more than 100,000 individual antenna dipoles, using powerful computers to process the radio signals as if it formed a ‘dish’ of 1900 kilometres diameter. LOFAR is unparalleled given its sensitivity and ability to image at high resolution (i.e. its ability to make highly detailed images).

This study was coordinated by Federica Govoni of the National Institute for Astrophysics (INAF) of Cagliari. Participants from the Netherlands were Emanuela Orrù, Marco Iacobelli, Roberto Pizzo and Timothy Shimwell from ASTRON, Huub Röttgering and Reinout van Weeren of the Leiden University, Michael Wise of the Netherlands Institute for Space Research (SRON) and researchers from different European Institutes (in Italy, Germany, France, Switzerland, Sweden, England).

Image: Composite image of the galaxy clusters pair Abell 0399 and Abell 0401. The system is at approximately 1 billion light-years from the Earth while the two galaxy clusters are about 10 millions light-years apart in projection. The cores of the two galaxy clusters are permeated by a high-temperature plasma that emits in the X-rays (red tones). Furthermore, observations in the microwaves shows a tenuous filament of matter connecting the two clusters (yellow tones). The low-frequency image in the radio waves (blue tones) reveals several bright discrete sources associated to individual galaxies and two diffuse radio halos towards the centers of the two clusters of galaxies. A striking ridge of radio emission is visible along the filament connecting Abell 0399 and Abell 0401 revealing the presence of a vast magnetic field illuminated by a population of high-energy electrons. Credits: DSS (optical), XMM-Newton (X-rays), PLANCK satellite (y-parameter), F. Govoni et al. 2019, Science (radio). Image by M.Murgia, INAF.