A ring of cold gas surrounds the supermassive black hole of the Milky Way

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A lot of things are happening in the center of our galaxy. A supermassive black hole named Sagittarius A-Star resides there, attracting materials with its inexorable gravitational attraction. In this hallucinating neighborhood, where the laws of physics are outdated, astronomers have detected a ring of cold gas.

Sagittarius A-Star, or Sag. A * abbreviated to 26 000 light years from Earth and about 4 million times more massive than our Sun. All this mass means an overwhelming gravitational energy that attracts matter to A *. But before the material is sucked, it resides in a rotating accretion disk.

Astronomers have known all this for a while. Sagging. A * is a bright X-ray source because the disc material is compressed and heated, which causes the release of X-rays. It is not just heated; overheated at 10 million degrees Celsius (18 million degrees Fahrenheit). The X-ray observatories in space made it possible to visualize all this at about a tenth of a year of light from the hole itself.

A view of Sgr A * and the supermassive black hole located 26 000 light-years from the Earth in the center of the Milky Way. Credit: Chandra Telescope, NASA.

But there is also a large amount of "cold" hydrogen within a few light-years of the hole. This gas is only cold in comparison to the other gas. It is about 10,000 degrees Celsius (18,000 degrees Fahrenheit). The contribution of this type of colder gas to the black hole atmosphere was unknown. Until now, anyway.

"We hope that these new observations from ALMA will help the black hole to reveal some of its secrets."

Elena Murchikova, lead author of the article, astrophysicist at the Institute for Advanced Study in Princeton, New Jersey.

At present, a new study using the Atacama (ALMA) large millimeter / sub-millimeter lattice has given us our first image of this cold gas body. Their article detailing this work appears in the June 6 issue of Nature. Here's how they did it.

There is enough radiation near the center of the galaxy for the hydrogen atoms to lose their electrons. Then they find them again, in a continuous cycle. In doing so, recombination releases energy in a particular millimeter wave length signal. This signal can travel to Earth with very little signal loss.

Three of the dishes that make up the Atacama Large Millimeter / Submillimter Array (ALMA). Image credit: H. Calderón - ALMA (ESO / NRAO / NAOJ)
Three of the dishes that make up the Atacama Large Millimeter / Submillimter Array (ALMA). Image credit: H. Calderón – ALMA (ESO / NRAO / NAOJ)

ALMA is a very fine, powerful and sensitive machine, able to tune this specific signal. Astronomers have used this power and sensitivity to produce the very first image of the cold gas disk at a distance of one hundredth of a light year from Sag. A*. Their observations allowed astronomers to map the location of this cloud and to follow its movement. They also measured the size of the hydrogen cloud, which contains about one-tenth of Jupiter's mass, or one-tenth of the Sun's mass.

"We were the first to image this elusive disc and study its rotation."

Elena Murchikova, lead author of the article, astrophysicist at the Institute for Advanced Study in Princeton, New Jersey.

The Doppler effect also comes into play. As the hydrogen recombination signal travels to Earth, its frequency moves to the blue part of the spectrum. By mapping these changes, astronomers could see that the gas ring was circling the black hole.

The ALMA image of the frozen hydrogen disc that circulates around the supermassive black hole in the center of our galaxy. The colors represent the movement of the gas with respect to the Earth: the red part moves away, so that the radio waves detected by ALMA are slightly stretched or displaced to the right.
The ALMA image of the frozen hydrogen disc that circulates around the supermassive black hole in the center of our galaxy. The colors represent the movement of the gas with respect to the Earth: the red part moves away, so that the radio waves detected by ALMA extend slightly or are shifted towards the "more red" part of the spectrum; the blue color represents the gas going towards the Earth, so that the radio waves are slightly contracted or displaced towards the "more blue" part of the spectrum. The reticle indicates the location of the black hole. Image credit: ALMA (ESO / NAOJ / NRAO), E.M. Murchikova; NRAO / AUI / NSF, S. Dagnello

As the environment around a black hole is very chaotic, it takes a lot of work to understand everything that goes on there. This information will help scientists understand this environment and understand how a black hole consumes material.

"We were the first to image this elusive disc and study its rotation," said Elena Murchikova, astrophysics member of the Institute for Advanced Study in Princeton, New Jersey, and lead author of the paper. "We are also probing the accretion in the black hole. It's important because it's our nearest supermassive black hole. Despite this, we still do not understand how this accumulation works. We hope that these new observations from ALMA will help the black hole to reveal some of its secrets. "

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