An article published in the journal “The Astrophysical Journal Letters” reports the results of observations of a structure similar to a chimney which functions as a sort of vent from which hot gas coming from Sagittarius A*, the supermassive black hole at the center of the Milky Way, escapes. A team of researchers combined observations conducted in X-rays with NASA’s Chandra Observatory and radio waves with the MeerKAT radio telescope to pinpoint that vent, which is located at about 700 light-years from the center of the Milky Way. This helps reconstruct the processes in which Sagittarius A* swallows materials and re-emits some of them.
The image (X-ray: NASA/CXC/Univ. of Chicago/S.C. Mackey et al.; Radio: NRF/SARAO/MeerKAT; Image Processing: NASA/CXC/SAO/N. Wolk) shows a region near the center of the Milky Way. Sagittarius A* is within the very bright area at the bottom. Blue clouds indicating X-rays detected by Chandra occupy much of the image. In some areas, the blue clouds appear to form spheres of teal light: these are dust halos that reflect X-rays from sources behind them. In the center, there’s a sort of pillar of blue light that comes from Sagittarius A*. That’s the chimney of hot gas surrounded by red clouds full of stars, visible as small red spots. Near the top of the pillar is a thin, sloping blue stripe, outlined by a gray box, which was interpreted as the vent of the chimney-like structure. In the left panel, there is a zoom of the area at the top center, observed by Chandra.
The Chandra X-ray Observatory had already made it possible to locate the chimney, in that case, in combination with ESA’s XMM-Newton space telescope. Now, more data collected by the MeerKAT radio telescope that detects radio emissions from the area around Sagittarius A* shows the effect of the magnetic fields that enclose the hot gas in that chimney.
Observations conducted for a period equivalent to approximately 12 days with Chandra allowed to obtain many details of the area near Sagittarius A*. This made it possible, among other things, to resolve many weak point sources, and this allowed them to be removed. The consequence is that it was possible to examine the diffuse emission and confirm that it’s produced by hot plasma. The image shows that the plasma spreads out, forming what appears to be a section of a channel through which the flow of hot gas could pass.
According to the researchers’ reconstruction, the vent was formed when the hot gas rising through the chimney hit the cold gas it encountered. The shock waves could be what generates the brightness of the walls of the pipeline in X-rays, compared to the sonic booms generated by planes passing at supersonic speed. On the left side of the vent, the brightness is even more intense, probably because the gas flowing upwards hits the wall at a more direct angle and with greater force.
The researchers think that the hot gas came from a sequence of events involving materials falling toward Sagittarius A*, and that eruptions from the supermassive black hole then pushed the gas up the chimney and out through the vent.
According to previous studies, every few centuries near the location of Sagittarius A* there are X-ray flares and they could play an important role in pushing the hot gas up through the vent. Powerful, explosive energy releases are estimated to occur approximately every 20,000 years. Most of the energy from these releases may rise through the chimney vent.
This study is useful for reconstructing the processes in which Sagittarius A* swallows materials and re-emits part of them. However, there’s still much to understand, and the researchers indicate that a useful future investigation would be to determine the velocity of plasma in chimneys. Gabriele Ponti of the Italian National Institute of Astrophysics, one of the authors of the article, talked about a calorimeter in the soft X-ray band that could be available in the future and would enable the measurement of properties such as the outflow velocity or turbulence of the plasma. This would help to better understand the processes taking place around Sagittarius A* and supermassive black holes in general and their possible influence on the surrounding environment, which is sometimes the entire galaxy that hosts them.
