Insight into an active galaxy core


Gas filaments generated by the black hole in the galaxy group Nest200047. (Image: University of Bologna)

At the heart of most galaxies are supermassive black holes. When they actively devour matter, they emit enormous amounts of hot gases, radiation, and fast particles. But how the emitted material will develop over time has so far been unclear. However, astronomers have now succeeded for the first time in reconstructing the development of such emissions over time with the help of radio and X-ray telescopes. It showed that the fragile-looking gas bubbles and filaments last astonishingly long – more than a hundred million years – and that they can reach far beyond their home galaxy.

Supermassive black holes and their galactic “hosts” are closely linked: How active the central black hole is determines how fast a galaxy grows or how actively it forms stars. If such a galaxy core devours a lot of matter in the form of dust or even stars, this releases enormous amounts of energy. Part of it escapes into space in the form of high-energy radiation, which can make active galaxy nuclei visible as quasars over billions of light years. At the same time, an active, supermassive black hole also releases particle streams that are almost as fast as light. The interaction of these jets with the surrounding matter and the magnetic fields create bubbles and filaments of hot, ionized gas that have a decisive impact on the galactic environment and star formation.

Cosmic gas formations in view

But how these emissions from an active black hole change over time and how long it takes, for example, for the newly formed gas formations to dissolve in the galactic and intergalactic medium, has so far been unclear. The reason: Until now, these ejections from the black hole could mainly be mapped and tracked using the synchrotron radiation of the electrons that are strongly accelerated in them. But because the particles in the gas formations lose their energy, this radiation also quickly subsides. “The most advanced phases of their development therefore remained undocumented and this prevents us from fully understanding how they are linked to the external medium and what feedback is there,” explain Marisa Brienza from the University of Bologna and her colleagues.

In the meantime, however, astronomy has new telescopes that are so powerful and sensitive that they can even capture the weaker radiation from the electrons in the older gas bubbles and filaments that arise around a black hole. For their study, the researchers used the radio antennas of the Low Frequency Array (LOFAR) in the Netherlands, one of the most powerful radio telescopes in the world in the low frequency ranges. With him, the team set their sights on the group of galaxies Nest200047, some 200 million light years away. At its center lies a particularly massive galaxy with an active supermassive black hole. By observing at 53 and 144 megahertz, the astronomers succeeded in making even the oldest ejections from this black hole visible. At the same time, they also aimed the eROSITA X-ray telescope at this galaxy and were able to observe, among other things, the X-rays that arise when the matter ejected from the black hole interacts with the intergalactic medium.

Stable for millions of years

Together, these observations enabled astronomers to reconstruct the fate of the hot gases emanating from the black hole over 100 million years. “Our observations show the late development of multiple generations of the bubbles emanating from an active galaxy nucleus in a group of galaxies in an unprecedented level of detail,” state Brienza and her team. The recordings revealed, among other things, how fresh bubbles of hot plasma are formed as mushroom-like protuberances near the black hole. This then creates bubbles and ring-shaped structures that migrate outwards, similar to gas bubbles rising in water. More detailed analyzes revealed that some of the bubbles observed in Nest200047 must be more than 100 million years old.

The X-ray data showed that even older remains of such bubbles in the form of thin gas filaments millions of light years long extend far beyond the galaxy of origin. “One would expect that these thin, long filaments would have long been destroyed by random, turbulent movements,” the researchers write. Above all, the contact with the rather turbulent intergalactic medium should actually disperse and dissolve these thin gas threads. “The fact that the bubbles and filaments have retained their integrity even after hundreds of millions of years and after moving for hundreds of thousands of light years, however, contradicts this.” The astronomers suspect that magnetic fields play a decisive role in these apparently fragile gas structures so long to keep unmixed.

Source: Marisa Brienza (Università di Bologna) et al., Nature Astronomoy, doi: 10.1038 / s41550-021-01491-0

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