There’s a lot going on at the center of our galaxy. A supermassive black hole named Sagittarius A-Star resides there, drawing material in with its inexorable gravitational attraction. In that mind-bending neighbourhood, where the laws of physics are stretched beyond comprehension, astronomers have detected a ring of cool gas.
Sagittarius A-Star, or Sag. A* for short, is 26,000 light years from Earth and is approximately 4 million times more massive than our Sun. All that mass means overwhelming gravitational energy draws material toward A*. But before material is sucked in, over the vent horizon, it resides in a rotating accretion disk.
Astronomers have known all about this for a while. Sag. A* is a bright x-ray source, because the material in the disk is compressed and heated, causing it to release x-rays. It’s not just heated; it’s super-heated to 10 million degrees Celsius (18 million degrees Fahrenheit.) X-ray observatories in space have been able to see all this down to about a tenth of light year from the hole itself.
But there’s also a vast amount of “cold” hydrogen gas within a few light years of the hole. This gas is only cold in comparison to the other gas. It’s about 10,000 degrees Celsius (18,000 degrees Fahrenheit.) What contribution to the black hole environment this cooler gas made was unknown. Until now, anyway.
Now a new study using the Atacama Large Millimeter/Sub-Millimeter Array (ALMA) has given us our first image of this cold body of gas. Their paper detailing this work appears in the June 6th issue of Nature. This is how they did it.
There’s enough radiation near the center of the galaxy to make hydrogen atoms lose their electrons. Then they regain them again, in an ongoing cycle. As they do so, the recombination releases energy in a particular millimeter-wavelength signal. This signal can travel all the way to Earth with very little signal loss.
ALMA is a very fine machine, powerful and sensitive, and it’s able to tune into this specific signal. Astronomers used that power and sensitivity to produce the first-ever image of the disk of cool gas at a distance of only one-hundredth of a light year from Sag. A*. Their observations let astronomers map the location of this cloud, and also to track its motion. They also measured the size of the hydrogen cloud, and it contains about one-tenth the mass of Jupiter, or one ten-thousandth the mass of the Sun.
The Doppler effect comes into play, too. As the signal from the hydrogen recombination travels to Earth, its frequency shifts to the bluer part of the spectrum. By mapping those shifts, the astronomers were able to see that the ring of gas is rotating around the black hole.
Since the environment around a black hole is so chaotic, it takes a lot of work to understand everything that’s going on there. This information will help scientists understand that environment, and how a black hole consumes matter.
“We were the first to image this elusive disk and study its rotation,” said Elena Murchikova, a member in astrophysics at the Institute for Advanced Study in Princeton, New Jersey, and lead author on the paper. “We are also probing accretion onto the black hole. This is important because this is our closest supermassive black hole. Even so, we still have no good understanding of how its accretion works. We hope these new ALMA observations will help the black hole give up some of its secrets.”