For fifty years, astronomers have been tracking a theoretical wind around Sagittarius A*, the supermassive black hole at the center of our Galaxy. Despite ever more powerful instruments, this signature remained elusive. Today, a team from Northwestern University has finally succeeded in detecting it.
To achieve this, the researchers used the ALMA network in Chile over five years. By applying an unprecedented calibration technique to dim the black hole's radio brightness, they obtained a map a hundred times more detailed of the surroundings. This image reveals a cone-shaped cavity about three light-years long, where cold gas has completely disappeared.
Scientists quickly understood that a hot wind blowing from the black hole must be the origin of this structure. By pushing away or heating the cold gas, it creates a void that cannot be explained by neighboring stars alone. Calculations show that the required energy far exceeds that provided by surrounding celestial bodies.
To confirm their discovery, the team compared their data with that from the Chandra space telescope. The observed X-ray emissions coincide perfectly with the cavity detected by ALMA. This concordance dispelled doubts: it was indeed the long-sought wind, with activity spanning at least 20,000 years according to observations.
Sagittarius A* thus reveals itself in a calm state, unlike many supermassive black holes in other galaxies that shine intensely. This discovery offers a unique view of the behavior of these objects in their most common phase, that of quiescence.
Map of the 12CO line (J = 2−1) in the inner parsec of the Milky Way.
The results, published in
The Astrophysical Journal Letters, mark a major advance. They show that even in its apparent tranquility, the heart of our Galaxy remains active and shapes its environment in a discreet but very real way.
How black hole winds shape galaxies
When a black hole swallows matter, part of it is ejected as winds or jets. These flows can blow interstellar gas hundreds of light-years away, slowing or stopping star formation in some regions. On a larger scale, they regulate the growth of the entire galaxy by redistributing energy and matter.
Winds are not always powerful. Some black holes, like Sagittarius A*, produce modest but enduring winds. Their cumulative effect over millions of years can nevertheless transform their surroundings. Understanding these mechanisms helps explain why some galaxies are more active than others.
Recent observations provide a missing piece: that of a wind in action around a quiet black hole. Theoretical models predict that this phenomenon is universal, but evidence was lacking. Now astronomers can refine their simulations by incorporating these data from our own Galaxy.