An international team of researchers, including Dr Simon Purser from the DIAS Astronomy and Astrophysics Section, observed the evolution of warm gas coming from an active black hole for the first time. The team were able to look at these structures that closely remind of the smoke streams produced by volcanic eruptions with unprecedented detail and on a scale of a hundred million years.

Their study published in Nature Astronomy focused on the Nest200047 system – a group of roughly 20 galaxies approximately 200 million light-years away. The central galaxy of this system houses an active black hole around which researchers observed many couples of gas bubbles diverse in age, some unknown filaments of magnetic fields and relativistic particles as big as hundreds of thousands of light-years.

These observations were possible thanks to LOFAR (LOw Frequency ARray), the largest low-frequency radio telescope in the world. LOFAR can intercept radiations produced by the oldest electrons that are currently detectable. This state-of-the-art tool stems from the great effort of nine European countries and allowed researchers to “go back in time” to more than 100 million years ago and retrace the activity of the black hole sitting at the centre of Nest200047.

“Our investigation shows how these gas bubbles accelerated by the black hole are expanding and transforming in time. Indeed, they create spectacular mushroom-shaped structures, rings and filaments that are similar to those originating from a powerful volcanic eruption on planet Earth”, states Marisa Brienza who is the first author of this study and a researcher at the Department of Physics and Astronomy “Augusto Righi” of the University of Bologna and a member of the Italian National Astrophysics Institute.

Bubbles made of particles

At the core of each galaxy sits a supermassive black hole (with a mass of several solar masses). The activity of such black hole crucially impacts the evolution of the galaxy and the intergalactic environment hosting it. For years researchers have been trying to figure out in which way and at which rate the action of these black holes produces those effects.

When active, black holes consume whatever surrounds them and, in that process, they release enormous quantities of energy. Sometimes this energy comes in the form of particle streams moving at the speed of light and producing radio waves. In turn, these streams generate bubbles of particles and magnetic fields that, by a process of expansion, can heat and move the intergalactic medium surrounding them. This has an immense influence on the evolution of the intergalactic medium itself and, as a consequence, on star formation rates.

This study purports that the impact of active black holes moves on scales that are up to 100 times bigger than the hosting galaxy and that such impact lasts up to hundreds of millions of years.

“LOFAR gave us a unique view over the activity of black holes and their effects on their surrounding environment”, explains Annalisa Bonafede, one of the authors of the study and a professor at the University of Bologna, as well as INAF member. “Our observations on Nest200047 crucially show how magnetic fields and very old particles accelerated by black holes play a central role in transferring energy to the outer region of groups of galaxies”.

For this study, researchers also exploited observations in the X band obtained using eROSITA, the new telescope hosted in the Russian-German space observatory “Spectrum-Roentgen-Gamma” (SRG) and managed by the Max Plank Institute for Astrophysics (Germany) and several Russian institutions, including Moscow Space Research Institute. X band data allowed researchers to better study the characteristics of the intergalactic medium surrounding the gas bubbles.

Gas filaments

These observations brought about other unexpected discoveries: thin gas filaments moving almost at the speed of light and magnetic fields as big as a million light-years.

According to the researchers, these filaments are the remnants of the bubbles that the Nest200047 black hole produced hundreds of millions of years ago and that are now shattering and mixing with the intergalactic medium. It is believed that studying these structures will lead to discovering new and important information about the physical features of the intergalactic matter and the physical mechanism regulating the energy transfer between the bubbles and the outer environment.

“In the future, we will be able to study the effects of black holes on galaxies and the intergalactic medium with increasing detail. Eventually, we will be able to unveil the nature of the filaments we discovered thanks to the angular resolution of LOFAR combined with the data from international LOFAR stations”, adds Gianfranco Brunetti, co-author of this study as well as an astrophysicist at the INAF Bologna and Italian coordinator of the LOFAR consortium.

LOFAR is managed by ASTRON, the Netherland Institute for Radio Astronomy, and is composed of thousands of antennas hosted by 51 radio stations scattered over different European countries. LOFAR can intercept the lowest frequencies of radio waves on Earth (between 10 and 240 mega-Hertz). Follow the activities of the Irish station of the LOFAR network, I-LOFAR, on their lofar.ie website.

Article access

Read the full article (“A snapshot of the oldest active galactic nuclei feedback phases”) online here.