A Magnetic Vortex in the Centre of the Milky Way

It takes light around 27,000 years to travel from the centre of the Milky Way to Earth. Scientists published an image of the black hole located there for the first time in 2022. The "Event Horizon Telescope (EHT) Collaboration", an association of more than 300 researchers from Africa, Asia, Europe, North and South America, was responsible for this.

Sagittarius A* is the scientific name of this object in the centre of the Milky Way - derived from the Latin term for Sagittarius, the constellation that is crossed by the band of the Milky Way at this point. The EHT group has now published new images of Sagittarius A* . They show strong and organised magnetic fields spiralling out from the edge of the supermassive black hole.

Great Similarities Between Two Black Holes

What makes this discovery so significant from a scientific point of view is that the structure of the magnetic field of Sagittarius A* looks strikingly similar to that of another black hole located in the centre of the galaxy M87. This suggests that strong magnetic fields are common to all black holes. Furthermore, this similarity suggests that Sagittarius A* is also emitting a jet - as is the case with M87* - which has not yet been detected. The results of these investigations have now been published in The Astrophysical Journal Letters .

The starting point of the study was the observation that although the supermassive black hole in the Milky Way is more than a thousand times smaller and lower in mass than that of M87, it looks remarkably similar to it. This prompted the scientists to ask whether the two objects have common features beyond their appearance. They focussed on the magnetic fields in the vicinity of the black holes. After all, previous studies had shown that magnetic fields in the vicinity of M87* enable the black hole to hurl powerful jets of material into the neighbourhood.

Images in Polarised Light

The team hoped to find answers to their questions by taking a closer look at Sagittarius A* - this time, however, in polarised light, i.e. light whose waves oscillate exactly in one plane and not, as is usually the case, in all possible planes. And indeed: the image taken in this way shows strong, twisted and organised magnetic fields that are strikingly similar to the polarisation structure of the much larger and more water-rich black hole M87*. These strong and organised magnetic fields are crucial for the interaction between the black holes and the matter surrounding them.

What sounds so simple was a major challenge in reality, as Sagittarius A* changes dynamically at a rapid pace and the image plane does not remain still. Capturing images of the black hole thus requires state-of-the-art instruments that go beyond what was used to observe the more stable source M87*. Therefore, the original image was created by stitching together several snapshots to account for the motion of Sagittarius A* .

Improved Technology Makes for Better Images

The research teams involved are delighted with the results. They say that these images and the associated data offer new possibilities for comparing and contrasting black holes of different sizes and masses. With further improvements in technology, they are likely to reveal even more secrets of black holes and their similarities or differences in the future.

The EHT has conducted several observations since 2017 and is expected to observe Sagittarius A* again in April 2024. Each year the images get better as the EHT deploys new telescopes, larger bandwidths and new observing frequencies. Upgrades planned for the next decade will enable high-resolution movies of the black hole, potentially revealing a hidden jet and allowing astronomers to observe similar polarisation features in other objects. In the meantime, extending the EHT into space will provide sharper images of black holes than ever before.

Voices From the University of Würzburg

Dr Christian Fromm is a junior research group leader at the Chair of Astronomy at Julius-Maximilians-Universität Würzburg (JMU). He has led several projects in the theory group at the EHT, which have developed numerical simulations to understand the physics of the object. Fromm considers the striking similarity between the magnetic field structure of M87* and that of Sagittarius A* to be remarkable - especially because it raises the possibility that, despite the differences in mass, size and environment, the physical mechanisms that control the feeding and jet ejection of a black hole are common to all supermassive black holes. "We can now use this result to improve theoretical models and simulations, which will lead to a better understanding of the effects on matter near the event horizon of a black hole," says the astrophysicist.

Since 2021, the Chair of Astronomy at JMU has been coordinating a Germany-wide network of researchers who have set themselves the goal of investigating the physical processes behind the jet phenomenon and black holes. The spokesperson for this DFG Research Unit "Relativistic Jets in Active Galaxies" is Professor Matthias Kadler. He says: "The German research landscape has developed extremely positively in recent years in the field of research into black holes and their jets. We are excellently positioned to utilise and operate upcoming new large-scale research instruments, for example in high-frequency radio astronomy, in a leading global network. The strong role of German groups in the EHT is a good example of this."

Further Information

The international EHT collaboration is working to capture the most detailed images of black holes ever taken by developing a virtual telescope the size of the Earth. Backed by significant international investment, the EHT is combining existing telescopes with novel systems to create a fundamentally new instrument with the highest angular resolution yet achieved.

The individual telescopes involved in the EHT observations in April 2017 were:

• the Atacama Large Millimetre/submillimetre Array (ALMA)
• the Atacama Pathfinder EXperiment (APEX)
• the 30 metre telescope of the Institut de Radioastronomie Millimetrique (IRAM)
• the James Clerk Maxwell Telescope (JCMT)
• the Large Millimetre Telescope Alfonso Serrano (LMT)
• the Submillimetre Array (SMA)
• the Arizona Submillimetre Telescope (SMT)
• the South Pole Telescope (SPT).

Since then, the EHT has added the Greenland Telescope (GLT), the IRAM Northern Extended Millimetre Array (NOEMA) and the UArizona 12-metre telescope on Kitt Peak to its network. The data were processed in the computers of the Max Planck Institute for Radio Astronomy in Bonn and the MIT/Haystack Observatory in Massachusetts (USA).

Links

Event Horizon Telescope (EHT)

EHT Observing Campaign 2017

Contact

Dr Christian M. Fromm, Institute for Theoretical Physics and Astrophysics JMU Wuerzburg, E-Mail: christian.fromm@uni-wuerzburg.de

Prof Dr Matthias Kadler, Institute for Theoretical Physics and Astrophysics JMU Wuerzburg, e-mail: matthias.kadler@uni-wuerzburg.de

International Contacts

Dr Mariafelicia De Laurentis, Deputy EHT Project Scientist, University of Naples Federico II, Italy, e-mail: mariafelicia.delaurentis@unina.it

Prof. Dr Geoffrey Bower, EHT Project Scientist, Institute of Astronomy and Astrophysics, Academic Sinica, e-mail: gbower@asiaa.sinica.edu.tw

Prof Dr Huib Jan van Langevelde, EHT Project Director, JIVE and University of Leiden, Netherlands, e-mail:langevelde@jive.eu