Radio emission confirms that a magnetic field spans intergalactic space

Think of the large-scale universe as a web: Filamentary structures—threads of galaxies, gas, and dark matter—crisscross space. Where they intersect, gravitationally collapsing material forms galaxy clusters that can then merge through some still-unknown process. Some of the energy released by cluster mergers helps power relativistic particles that spiral around magnetic field lines.

One way that astronomers observe what happens during a merger is by detecting emissions that come from superheated plasma known as the intracluster medium (ICM). The density of the ICM is so low that the only way for matter to interact is through collisionless dealings between the plasma’s electric and magnetic fields. X rays are emitted from the interaction of the plasma’s electric field with free electrons; synchrotron radiation is emitted from cosmic-ray particles traveling through magnetic fields. (See the article by Lawrence Rudnick, Physics Today, January 2019, page 46 .)

Astronomers have long wondered whether the radio emission from magnetic fields in clusters extends to the filamentary structures, which contain about half of the universe’s baryons. The detection of emission there would be a first step toward understanding the physical processes that affect those baryons, and it would confirm the cosmic web structure of the universe.

The effort to find emission associated with filamentary structures has come up short so far, although the presence of filaments has been inferred from the detection of UV absorption lines (see Physics Today, July 2008, page 12 ). Radio sources extending beyond the ICM of galaxy clusters haven’t been detected by low-frequency radio telescopes because of insufficient sensitivity and calibration difficulties. Searches in the soft-x-ray spectrum await more sensitive satellite observations.

Now radio synchrotron emission has been newly detected between two merging galaxy clusters by the Low Frequency Array (LOFAR) telescope. It was built and designed and is operated by the Netherlands Institute for Radio Astronomy with contributions from international partners. A collaboration led by Federica Govoni of Italy’s National Institute for Astrophysics has found that the synchrotron emission arises from a magnetic field that extends over distances greater than previously thought possible. ¹

A European-sized radio dish

Measurements made before the new LOFAR observations led Govoni and her colleagues to suspect that a filamentary structure exists between the merging galaxy clusters Abell 0399 and Abell 0401. Figure 1 shows the clusters’ x-ray emission collected by the European Space Agency’s (ESA’s) XMM-Newton observatory. Microwave photon-intensity measurements around and between the clusters can help unravel their history and were collected by ESA’s Planck satellite (see Physics Today, June 2015, page 20 ). But without detecting radio emissions, astronomers couldn’t conclusively say whether a filamentary structure and magnetic field existed between the two clusters.

Figure 1.

LOFAR was designed to detect ultralow-frequency radio emissions in the range of 10–250 MHz (see Physics Today, March 2011, page 24 ). The telescope uses 25 000 electronically linked, digital dipole antennas in stations spread across France, Germany, Ireland, Italy, Latvia, the Netherlands, Poland, Sweden, and the UK. Each antenna is essentially a metal pole in the ground with some mesh wire underneath it. That design “was very cost efficient and that’s one of the greatest attributes of LOFAR,” says Amanda Wilber, a LOFAR astronomer from the University of Hamburg in Germany not involved with Govoni’s research.

Individual antennas far from each other create a long baseline that enables LOFAR to detect the faint radio energy emitted by distant planets, stars, galaxies, and clusters. Such a baseline can be used to produce sharp, highly resolved images. In the Netherlands, the antennas are spaced closely together, which provides an extended picture of the sky. The Dutch stations of the LOFAR array, one of which can be seen in figure 2, provided Govoni and her colleagues with sufficient resolution and sensitivity to detect the broad radio emission coming from the extended magnetic field.

Figure 2.

Before the researchers could make the successful detection, they had to synchronize and calibrate all the antennas. “It’s one of the most challenging parts for not only LOFAR but [all] low-frequency radio observations,” says x-ray astronomer Hiroki Akamatsu of the Netherlands Institute for Space Research. Charged particles in Earth’s ionosphere emit energy at frequencies that interfere with radio-astronomy telescopes. But in 2016 a team led by Reinout van Weeren of Leiden University in the Netherlands developed an effective calibration method for the LOFAR telescope that entails modeling the ionosphere and then removing its contribution from the radio-emission observations. ²

With the new calibration, the LOFAR community “can basically limit the effects of the ionosphere and see what is really happening above the atmosphere of the Earth,” says Wilber. “We made a lot of progress, and now we have very reliable results.”

The new LOFAR findings, the 140 MHz synchrotron emission shown in figure 1, provide clear evidence for the previously unseen magnetic field. Astronomers have observed radio relic emissions, which originate from individual galaxies and galaxy clusters. But the LOFAR detection is the first to observe the emission connecting two distant clusters. The magnetic field extends 3 megaparsecs (about 10 million light-years) from the clusters, far enough that the emission cannot be mistaken for a radio relic. Using similar magnetic field properties of already identified clusters, Govoni and her team estimate that the field strength is less than 1 μG, or about one-millionth the strength of Earth’s magnetic field.

Shocks to the system

Electrons in the magnetic field lose energy to synchrotron radiation and inverse Compton scattering over time. First-order calculations suggest that the electrons in the filament can therefore only travel at relativistic speed for 230 million years. At that rate, the maximum distance they could travel would be 0.1 Mpc, about one order of magnitude less than the length of the observed radio bridge. To get across the filament and its magnetic field, some mechanism acting along the entire cosmic-sized filament must accelerate the relativistic electrons to give them an extra boost.

To identify such a mechanism, one of Govoni’s coauthors, Franco Vazza of the University of Bologna in Italy, performed magnetohydrodynamic numerical simulations. ³ In the model, the particles in the filamentary structure are energized by shock waves produced from merging clusters. The first simulation accelerated a population of relatively young electrons to not-quite-relativistic speed for 230 million years. But the associated radio emission was about one-thousandth as intense as the LOFAR observations.

Electrons need a GeV of energy to radiate detectable synchrotron emission at radio frequencies. The shock waves were too small and inefficient to get the job done in the first simulation. The second one reasonably reproduced the LOFAR intensities by including an additional, preexisting population of billion-year-old relativistic electrons that get accelerated by stronger merger-induced shock waves.

Several details about the mechanism still need to be worked out. Govoni and her colleagues do not know yet where the old relativistic electrons come from nor what mechanism fills the filament with them. “It is important to understand whether the emission detected in the filament that connects Abell 0399 and Abell 0401 is a common phenomenon in the cosmic web,” says Govoni. “I hope that astronomers will soon find other magnetized filaments.”