Structure of the supermassive black hole at the center of our galaxy revealed

We know that most galaxies harbor a supermassive black hole at their center, and that these objects can influence both their immediate environment and the evolution of galaxies. And ours, the Milky Way, is no exception: in its central regions, 26,000 light years from Earth, is Sagittarius A*, a black hole with a mass of about four million suns. Sagittarius A* (Sagittarius A Star or SgrA*) is the closest supermassive black hole to Earth, and is the ideal candidate to study what happens in the vicinity of these objects. A study published today reveals that its intrinsic structure is almost circular at 1.3 and 0.7 centimeter wavelengths.

In 2020, the Nobel Prize in Physics has awarded to the studies of Sgr A*, which have proven the compact object at the center of our Galaxy as a supermassive black hole. However, SgrA* also shows different properties with the other supermassive black holes. For instance, its emission is very weak and its ability to convert matter into energy is up to hundreds of times less than in other more massive black holes. And there is still no clear evidence that it has a jet, or a flow of material that emerges from both poles at very high speed, as it happens in the brightest active galaxies.

"There is a long debate about the origin of the energy we observe in Sgr A*: some studies suggest that its radio-dominant emission is due to the accretion flow, while others suggest that it comes from a jet", says Ilje Cho, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who leads the study. 

But studying SgrA*, despite its proximity, is not easy: our Solar System lies in the plane of the Milky Way, and in that direction there are large amounts of gas and dust. The dust limits observations to radio waves, infrared and X-rays, which can pass through it, but still suffer from the scattering caused by the gas clouds.

To resolve the fine structure of SgrA*, the science team used the very long baseline interferometry (VLBI) technique, which involves the synchronised use of numerous geographically separated radiotelescopes to create a virtual telescope the size of the distance between the telescopes. In addition, they applied the scattering model from recent studies based on the historical observations to their observations with the East Asian VLBI Network (EAVN).

"Before correcting the effect of scattering from the interstellar medium, the structure of Sgr A* was elongated towards the East-West direction. Previous studies also pointed to a similar structure, but with our work we show that the elongation is mainly due to the scattering effect. Three independent methods confirm that Sagittarius A* has a circular structure", says Ilje Cho (IAA-CSIC).

The circular shape of Sgr A* would imply that the rotation axis of the flow is almost pointing towards us (although the current data do not entirely rule out the possibility that the dominant energy of Sagittarius A* comes from a jet). Furthermore, the team has found that, in addition to thermal energy, produced by the falling and heating of material towards the black hole, there is also non-thermal energy, emitted by accelerated particles, perhaps from the magnetic field. "Although the mechanism of the non-thermal process has not been investigated in this study, it is important to take it into account in order to build a reliable theoretical model of how SgrA* absorbs matter," says Ilje Cho (IAA-CSIC).

The study was carried out with the East Asian VLBI Network (EAVN), which consists of twenty-one radio telescopes: six in China, eleven in Japan and four in Korea. For this study, ten and eight of them have been used to observe at wavelengths of 1.3 centimetres and 7 millimetres respectively. They are also part of the multi-wavelength campaign of the Event Horizon Telescope (EHT), which produced the first image of a black hole, that of the galaxy M87.

This work has also found a relationship between the size and brightness of SgrA* and the observing wavelength. Assuming the same relationship at shorter wavelengths, the size and brightness of Sgr A* is predicted at a wavelength of 1.3 millimetres. "This could be of great help for the analysis of the Event Horizon Telescope (EHT) data in order to make the first image of the shadow of the black hole in Sagittarius A*", says Guang-Yao Zhao, a researcher at IAA-CSIC and corresponding author of the work. "High-resolution imaging of Sgr A* is challenging in many ways, so results at lower frequencies are really essential to overcome the difficulties", adds José Luis Gómez, researcher at the IAA-CSIC and head of the EHT group at the IAA.