Radio telescope reveals previously unseen synchrotron emission

Think of the universe as a web: Filamentary structures—threads of galaxies, gas, and dark matter—stretch across space. Where they intersect, galaxy clusters form by accreting material or from a merging of two or more clusters. (See the article by Lawrence Rudnick, Physics Today, January 2019, page 46 .) Some of the energy released by cluster mergers boosts relativistic particles that spiral around magnetic field lines. Astronomers have long wondered whether the radio emission produced by mergers extends beyond the clusters themselves along the filamentary structures.

The answer has so far eluded astronomers, because of insufficient sensitivity and calibration difficulties associated with low-frequency radio telescopes. Now new observations from the Low-Frequency Array (LOFAR) telescope network have detected radio synchrotron emission between the merging Abell 0399 and Abell 0401 galaxy clusters. A collaboration led by Federica Govoni of Italy’s National Institute for Astrophysics has found that the emission arises from relativistic electrons and a magnetic field that’s larger than previously thought possible.

The magnetic field was detected from LOFAR measurements of radio emission (blue in the figure) between two clusters, illuminated by x-ray measurements (pink) from the European Space Agency’s XMM-Newton. The faint yellow around the clusters comes from measurements made by the ESA’s Planck satellite. (The finding is the result of the Sunyaev–Zeldovich effect, which arises from photons that experience inverse Compton scattering when traveling through the galaxy clusters.) First-order calculations suggest that the electrons travel at relativistic speed for 230 million years, which means the maximum distance they could travel would be 0.1 megaparsecs (300 000 light-years). But the bridge of radio emission is about 3 Mpc long, so some mechanism must give the relativistic electrons an extra boost.

To figure out the mechanism, the researchers employed magnetohydrodynamic simulations. The model that most agreed with observations reaccelerated a population of billion-year-old relativistic electrons using shock waves generated by the merging of the two galaxy clusters. (F. Govoni et al., Science 364, 981, 2019 .)