At the very center of the Milky Way lies a giant—Sagittarius A*, a supermassive black hole that behaves unusually calmly compared to similar objects in other galaxies. Unlike its "siblings," it does not emit powerful relativistic jets of plasma, known as jets. Nevertheless, astrophysicists have long suspected that even such a "quiet" black hole should occasionally display activity in the form of hot winds escaping from the vicinity of its event horizon. After more than half a century of searching, scientists have finally managed to observe this phenomenon, which may help explain the mysterious passivity of our black hole.

Features of Sagittarius A*

Sagittarius A* is the gravitational center of our Galaxy, with a mass exceeding that of the Sun by four million times. Despite its size, this object is extremely calm by cosmic standards. Unlike the active nuclei of other galaxies, such as quasars, whose radiation outshines the combined light of all their stars, the surroundings of Sagittarius A* show little activity and do not produce jets visible across billions of light-years.

Theoretical Background and Search

As early as the 1970s, scientists proposed that even inactive black holes could experience bursts of activity. The accretion disk and magnetic fields are capable of triggering powerful outflows and generating hot winds. Gas and dust falling into the black hole accelerate to tremendous speeds and heat up, but some of this material is not absorbed—instead, it is ejected back into space as a hot wind.

It is believed that such winds can transfer energy and momentum into the interstellar medium, heating or expelling gas, thereby regulating the black hole’s growth and influencing the rate of star formation around it.

Challenges of Observation

For a long time, detecting a hot wind near Sagittarius A* was impossible due to the extremely complex region around the black hole—the so-called circumnuclear disk. This is a dense cluster of stars, dust, and gas that effectively hides the processes occurring near the event horizon.

A Breakthrough in Research

American astrophysicists Mark Gorski and Elena Murchikova from Northwestern University in Illinois used the ALMA radio telescope in Chile and spent five years collecting data on the inner part of the circumnuclear disk. To isolate the faint glow of cold gas against the bright, flickering emission of the black hole itself, the researchers created a computer model simulating this flickering and then "subtracted" it from the overall signal. As a result, they obtained a map of cold gas within a few light-years of Sagittarius A*, with a resolution 100 times greater than previous observations.

The first surprise was the discovery of a large amount of cold gas in the immediate vicinity of the black hole, where it was previously not expected to be found in significant volumes.

A Key Discovery

On the resulting map, the scientists noticed a distinct conical region almost devoid of cold matter—like a tunnel or hole in the disk’s gas structure. Comparing these data with X-ray observations, which detect emission from hot material, the researchers found a striking match: the empty cone perfectly coincided with a region filled with hot gas. Thus, astronomers for the first time observed clear evidence of a hot wind breaking through the colder environment.

Calculations showed that the energy required for the hot gas to pass through this cone is equivalent to the output of 25,000 Suns. Neither nearby stars nor possible remnants of ancient supernovae could provide such power. The source of the hot wind is the processes occurring in the black hole’s accretion disk.

The Significance of the Discovery

This discovery significantly expands our understanding of how black holes operate. Previously, astronomers had already found giant structures above and below the plane of the Galaxy—the so-called Fermi bubbles, which are considered evidence that Sagittarius A* was much more active in the past and could have produced powerful jets.

The new data on the hot wind help explain the current "lull" of the black hole. The wind acts as a kind of exhaust valve: it blows material out of the vicinity of Sagittarius A*, depriving it of "fuel" and preventing it from entering a more active phase with jet formation.

Prospects for Further Research

The results obtained will help us better understand how Sagittarius A* rotates. For a long time, it was believed that the axis of rotation should be perpendicular to the plane of the Milky Way, but the first images of the black hole’s shadow, published in 2022, showed that the object is facing observers. At that time, there was not enough data for a precise analysis.

New observations may reveal the direction in which matter is moving as it falls into the black hole. This is important not only for understanding the black hole itself but also for studying the evolution of our Galaxy.

The research results have been published in the electronic archive of scientific articles and preprints of Cornell University.