Strange Ring of Gas Detected Around Our Galaxy’s Supermassive Black Hole

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Astronomers looking into the dark core of our galaxy saw a large, rotating disc of cool gas surrounding the supermassive black hole that lives therein. A strange ring of gas detected around our galaxy’s supermassive black hole. This disc has been hypothesized for a long time. It is now disclosed in all its turbulent glory.

“We were the first to picture this elusive disk and study its rotation,” said Princeton University’s astrophysicist Elena Murchikova. There are many types of galactic nuclei at the center of most galaxies, the supermassive black holes. Some blaze brilliantly as they spin gas, spinning into space billions of light-years of electromagnetic radiation.

Some are much slower, as the Sagittarius A * core of the Milky Way, a supermassive black hole tipping the scales four million times the Sun’s mass. But that doesn’t imply that the region around it is still and calm. We understand that Sgr A * slowly accretes material from the space around it. That material is likely to swirl around like a disc of accretion, like water around a drain.

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Until now, however, astronomers have only been able to glimpse the glowing warm part of it. An approximately spherical flow that sprays an X-ray image at a temperature of about 10 million Kelvin owing to frictional forces. But it shows no apparent indications of rotation, nor is it the flattened disc that we were expecting.

art imp of disc- strange ring of gas detected around our galaxy’s supermassive black hole
Strange ring of gas detected around our galaxy’s supermassive black hole. Artist’s impression of the disc. (NRAO/AUI/NSF; S. Dagnello)

But other telescopes have identified a cooler region of hydrogen gas (comparatively speaking) in the space beyond this warm gas. Extending from the black hole to a radius of about 6.5 light-years: only about 10,000 Kelvin. However, the function of this gas played in the process of accretion was uncertain.

But black hole radiation constantly ionizes the gas, causing the atoms of hydrogen to lose and recover their electrons. This provides off a weak radio signal -detected by astronomers using the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile. Then this radio signal was compiled into a picture. One that indicated the rotation of the disc obviously.

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disc-1-strange ring of gas detected around our galaxy’s supermassive black hole
(ALMA (ESO/NAOJ/NRAO), E.M. Murchikova; NRAO/AUI/NSF, S. Dagnello)

If you look at the above picture, you can see it. Sgr A * is represented by the white cross. The wavelengths are stretched, moved to the red end of the electromagnetic spectrum or redshifted in one portion of the disc-shown in red. This implies that the radiation moves away from us.

The reverse is happening in the blue portion. The wavelengths are compressed, moved to the blue end of the electromagnetic spectrum, or blueshifted. This implies that the radiation moves towards us.

Put together both halves and you have got a fairly strong rotation image there.

This radio signal also enabled the team to calculate the density and hence the gas mass in this region. And it is very tenuous, just between 0.0001 and 0.00001 times the Sun’s mass, spread over light-years of space.

The red oval, with a radius of 6.5 light-years, contains the molecular gas. (Murchikova et al., Nature, 2019)

In fact, Sgr A * is a fairly little eater. Based on these limitations, every year just over half of the dwarf planet Ceres mass of cool hydrogen gas drops into the black hole.

“This is our nearest supermassive black hole,” said Murchikova.

“Nevertheless, we still don’t have a strong knowledge of how its accretion operates. We hope these fresh ALMA observations will help the black hole to give up some of its secrets.”

The study was published in Nature.


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