Astronomers discover a ring around the supermassive black hole of the Milky Way

Artist impression of a ring of cold interstellar gas surrounding the supermassive black hole in the center of the Milky Way. The new ALMA observations reveal this structure for the first time. Credit: NRAO / AUI / NSF; S. Dagnello

After decades of study, astronomers have developed a clearer picture of the chaotic and crowded neighborhood surrounding the supermassive. black hole in the center of the Milky Way. Our galactic center is located about 26 000 light-years from Earth and its supermassive black hole, called Sagittarius A * (a "star"), represents 4 million times the mass of our Sun.

We now know that this region abounds with moving stars, interstellar dust clouds and a vast reservoir of gases both extraordinarily hot and relatively colder. It is expected that these gases gravitate around the black hole of a vast accretion disk that extends over a few tenths of a light year from the horizon of the black hole events.

Until now, however, astronomers have been able to visualize only the tenuous and warm part of this accumulator gas flow, which forms a roughly spherical flow and shows no obvious rotation. Its temperature is estimated at 10 million degrees (18 million degrees Fahrenheit), about two-thirds of the temperature found in the heart of our Sun. At this temperature, the gas shines violently in the X-ray light, allowing it to be studied using space-based X-ray telescopes, on a scale of 30 ° C. about a tenth of a year light from the black hole.

In addition to this hot and glowing gas, previous observations with millimeter-scale telescopes have detected a large, relatively cold (about 10,000 degrees Celsius or 18,000 degrees Fahrenheit) hydrogen reserve a few light years away from the black hole. . The contribution of this colder gas to the accretion flux on the black hole was previously unknown.

ALMA image of the cold hydrogen disk circulating 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. Credit: ALMA (ESO / NAOJ / NRAO), E.M. Murchikova; NRAO / AUI / NSF, S. Dagnello

Although our black hole in the galactic center is relatively silent, the radiation around it is strong enough for the hydrogen atoms to continually lose and recombine with their electrons. This recombination produces a distinctive signal of a millimeter wavelength, able to reach the Earth with very little loss en route.

With its remarkable sensitivity and powerful ability to see the finest details, the Atacama Large Millimeter Array / submillimeter ArrayALMA) was able to detect this weak radio signal and produce the very first image of the coldest gas disk at only one hundredth of light-year (about 1000 times the distance from Earth to the Sun) of supermassive black hole. These observations allowed astronomers to map the location and track the movement of this gas. The researchers estimate that the amount of hydrogen contained in this cold disc is about one-tenth of Jupiter's mass, or one-tenth of the Sun's mass.

By mapping the changes in the wavelengths of this radio light due to the Doppler effect (the light of the objects moving towards the Earth is slightly shifted towards the 'bluer' part of the spectrum, while the light from the objects moving away is slightly shifted to the 'more red' part), astronomers could clearly see that the gas was spinning around the black hole. This information will help to better understand how black holes devour the material and the complex interaction between a black hole and its galactic neighborhood.

"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. "

Publication: E.M. Murchikova, et al., "A cold accretion disk around the black hole of the galactic center," Nature, June 6, 2019