Previously accelerated by black holes, the particles are reaccelerated by shock waves resulting from the collision. The phenomenon helps scientists understand the structure of the universe on the largest scale.

A cosmic phenomenon on a colossal scale, resulting from the acceleration of a gas cloud by a black hole and its reacceleration by the shock waves from the merging of two galaxy clusters, has been observed, described and interpreted by an international collaboration of astronomers that included three Brazilians: Felipe Andrade-Santos, Vinicius Moris Placco, and Rafael Miloni Santucci.

All three, together with colleagues from other countries, are co-authors of the article "The case for electron reacceleration at galaxy cluster shocks", published in January 2017 in the journal Nature.

"The electrons that make up the cloud initially bounce off the supermassive black hole at the center of one of the galaxies and accelerate as a result. They are then reaccelerated by shock waves that propagate in the galaxy cluster when it collides with another cluster," Andrade-Santos said.

Galaxy clusters grow through the gravitational accretion of matter and by merging with other clusters and groups of galaxies. These mergers, which typically occur at faster-than-sound speeds, generate shock waves that propagate through the clusters for hundreds of millions of years and may reaccelerate particles previously accelerated in the galactic nuclei, where supermassive black holes are found.

The authors of the study investigated a collision between clusters Abell 3411 and Abell 3412, located some 2 billion light-years from Earth. Both are huge, extending for millions of light-years, and very massive: each is about one quadrillion times the mass of our Sun. But the particle gas of which the clusters are made up is extremely rarefied, more so than any vacuum produced in a laboratory on Earth, with a density in the range of 10-3 to 10-2 particles per cubic centimeter.

"During the formation of a cluster, collisions between gas particles raise the temperature of the medium to around 100 million degrees Celsius," Andrade-Santos said.

Owing to the extremely high temperature of the gas, the speed at which sound propagates in the medium is on the order of 1,000 km per second, almost 3,000 times faster than the speed of sound in Earth's atmosphere (343 m per second at sea level and 20 degrees Celsius). But galaxy clusters can collide at twice or three times this speed (2,000-3,000 km/s), hence giving rise to the shock waves that reaccelerate the particles.

The reacceleration of previously accelerated particles makes them emit electromagnetic radiation in the radio-frequency band. This radio emission was a mystery that had challenged astronomers for nearly two decades. The puzzle has now been solved by the research in question.

"Radio emissions from the regions in which clusters collide were first detected almost 20 years ago, but no one was able to explain how electrons could be accelerated to the point where they emitted radiation in this frequency band. An attempt was made to build a model in which the gas was compressed by the shock, so that the particles gained energy, but the mathematics didn't work out because the particles would have to gain far more energy from the shock than was predictable on the basis of astronomical observations," Andrade-Santos said.

"We worked with the hypothesis that a population of high-energy electrons already existed and would need only one last 'push' to start emitting radio waves. This was confirmed by our study. Observations of the shock between this pair of clusters showed that the radio emission was connected to the galaxy's jet, so clearly the electrons must have been initially accelerated by the black hole and then reaccelerated by the shock waves.

"Analyzing the radio emission in detail, we realized the electrons lost energy along the jet and were re-energized in the region of the shock. The novelty in our research is the discovery of a physical connection between the two phenomena. Double acceleration makes the particles a million times more energetic, boosting them from the level of a kiloelectron volt (keV) to the level of a gigaelectron volt (GeV)."

To reach this result, the researchers compiled data collected by a formidable array of equipment. The Chandra space telescope, which operates in the X-ray band, provided the location of the collision between the two clusters. The Giant Metrewave Radio Telescope (GMRT), which is installed near Pune in India and operates in the radio band, pointed to the link between the radio emission and a galaxy by locating the origin of the electrons. The Southern Astrophysical Research Telescope (SOAR), a visible-light and infrared telescope installed on Cerro Pachón in Chile, provided the data required to calculate the distance of the emitting galaxy, showing that it in fact belonged to one of the clusters. Japan's Subaru visible-light and infrared telescope, located on the summit of Mauna Kea in Hawaii, provided optical imaging of the galaxies. The two-telescope visible-light and infrared W. M. Keck Observatory, also on Mauna Kea, provided the spectra for the galaxies identified by Subaru.

Placco's and Santucci's contributions were vital to the collection and processing of the data obtained through SOAR. Their participation in this and other observatories in Chile has changed the profile of Brazilian astronomy, with strong support from FAPESP.

The study enriches scientists' understanding of the universe on the largest scale. The universe is known to consist of huge voids surrounded by gigantic filaments making up the cosmic web. These filaments are made of gas clouds and galaxies. Galaxy clusters are located at the intersections of filaments, which are like knots in the web. Clusters grow through the accretion of gas from these large-scale filaments and through mergers with other clusters and groups of galaxies. These mergers produce shock waves, which propagate through the clusters, reaccelerating particles previously accelerated by supermassive black holes in the galactic nuclei.

"Our work has two direct implications. The first is that simulations of galaxy clusters should include this population of high-energy electrons. The second is that laboratory experiments designed to simulate conditions in clusters should take these relativistic particles into account. In the future, we'll detect more cases like the one described in the study and acquire a better understand of the details. This is the first case, but with the advent of more powerful instruments, we'll be able to measure the phenomenon in other clusters," Andrade-Santos said.

This was a reference to new projects, such as the planned X-ray telescope Lynx, a possible successor to the Chandra space telescope. If the Lynx mission concept wins approval and is funded by NASA, it will vastly increase scientists' capacity to observe shocks between galaxy clusters, making important contributions to their mapping of the universe.