There is a supermassive black hole of nearly 4 million solar masses at the center of our Milky Way Galaxy. A large reservoir of hot and cooler hydrogen surrounds it within a few light-years. Now an international team of astronomers has used the Atacama Large Millimeter/submillimeter Array (ALMA) to image the cooler portion of this accreting gas.
An artist’s impression of a disk of cool interstellar gas surrounding the supermassive black hole at the center of the Milky Way. Image credit: NRAO / AUI / NSF / S. Dagnello.
Milky Way’s central region is brimming with roving stars, interstellar dust clouds, and a large reservoir of both phenomenally hot and comparatively colder gases.
These gases are expected to orbit the supermassive black hole, Sagittarius A*, in a vast accretion disk that extends a few tenths of a light-year from the black hole’s event horizon.
Until now, however, scientists have only been able to image the tenuous, hot portion of this accreting gas, which forms a roughly spherical flow and showed no obvious rotation. Its temperature is estimated to be a blistering 18 million degrees Fahrenheit (10 million degrees Celsius).
At this temperature, the gas glows fiercely in X-ray light, allowing it to be studied by space-based X-ray telescopes, down to a scale of about a tenth of a light-year from the black hole.
In addition to this hot gas, previous observations with millimeter-wavelength telescopes have detected a vast store of comparatively cooler (nearly 18,000 degrees Fahrenheit, or 10,000 degrees Celsius) hydrogen within few light years around the black hole. The contribution of this cooler gas to the accretion flow onto the back hole was previously unknown.
ALMA image of the disk of cool hydrogen gas flowing around the supermassive black hole at the center of our Galaxy. The colors represent the motion of the gas relative to Earth: the red portion is moving away, so the radio waves detected by ALMA are slightly stretched, or shifted, to the ‘redder’ portion of the spectrum; the blue color represents gas moving toward Earth, so the radio waves are slightly scrunched, or shifted, to the ‘bluer’ portion of the spectrum. Image credit: ALMA / ESO / NAOJ / NRAO / E.M. Murchikova / AUI / NSF / S. Dagnello.
Although Sagittarius A* is relatively quiet, the radiation around it is strong enough to cause hydrogen atoms to lose and recombine with their electrons continually. This recombination produces a distinctive millimeter-wavelength signal, which is capable of reaching the Earth with minimal losses on the way.
With its remarkable sensitivity and powerful ability to see fine details, ALMA was able to detect this faint radio signal and produce the first-ever image of the cooler gas disk surrounding Sagittarius A* at only about a hundredth of a light-year away, or about 1,000 times the distance from the Earth to the Sun.
These observations enabled the astronomers both to map the location and trace the motion of this gas.
They estimate that the amount of hydrogen in this cold disk is about one-tenth the mass of Jupiter, or one ten-thousandth of the mass of the Sun.
“We were the first to image this elusive disk and study its rotation,” said team leader Dr. Elena Murchikova, a researcher at Caltech and the Institute for Advanced Study.
“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.”
The research was published in the journal Nature.
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Elena M. Murchikova et al. 2019. A cool accretion disk around the Galactic Centre black hole. Nature 570: 83-86; doi: 10.1038/s41586-019-1242-z
