An accretion disk around the black hole of the Milky Way has been spotted

Some supermassive black holes announce their presence with very hot orbiting gas discs. But the monster in the center of the Milky Way was shy and wise. Now, astronomers have finally spotted the weakly shining accretion disk of the black hole, composed of infallible materials, long suspected but never seen before.

"I was very surprised to see it," said astrophysicist Elena Murchikova at the Institute for Advanced Study in Princeton, New Jersey. The disc was observed with the Atacama Large Millimeter Array, or ALMA, in northern Chile, researchers report. June 6 Nature.

The supermassive black hole of the Milky Way, called Sagittarius A *, is a monster of 4 million solar masses. But while some black holes gobble up the gas and dust around them, Sgr A * chooses softly. Such "malnourished" black holes "do not have enough food" for the surrounding gases to shine, says Murchikova.

The reduced glow of the disk partly explains why the scientists of the Event Horizon telescope were able to capture an image of the central black hole of the more distant galaxy M87, but not yet of Sgr A * (SN: 27/04/19, p. 6).

Previously, scientists had observed a cloud of hot gas (about 10 million kelvin) emitting high-energy X-rays around Sgr A *, as well as stars and gas clouds surrounding the black hole. But these gas sources did not seem to be organized into an orderly disk in orbit. Murchikova and his colleagues focused their research on colder gas, about 10,000 kelvins, about 280 billion kilometers from Sgr A *. She explains that watching only hot gas is like trying to study the Earth's climate by focusing on summers in the desert. "Both types of gas should fall into the black hole," says Murchikova. "You need a complete picture."

ALMA measured the cooler gases by observing light particles in a particular wavelength. These photons are emitted when electrons and protons in the gases combine to form hydrogen atoms. When Murchikova and his colleagues examined the distribution of photons around the black hole, they saw an oblong disk with a slit in the middle where the black hole is.

On one side of the disc, the wavelength of light was stretched or shifted to red. On the other hand, the light was crushed or moved to blue. This discovery means that one side of the disc is moving towards Earth and the other is moving away, clearly indicating that the disc is rotating.

"I would never have thought that I could actually attend such an organized rotation," says Murchikova.

The team also estimated the mass of the disc – between 0.00001 and 0.0001 times the mass of the sun, depending on the thickness of the disc. And the researchers estimated the amount of material falling in the black hole, about 2.7×10.^-ten solar masses per year, about half the mass of the dwarf planet Ceres.

"I think it is very exciting," says astrophysicist Anna Ciurlo of UCLA, who has not participated in this new work. His team used the Keck telescope from Hawaii to look for signs of the disc in infrared wavelengths, but found nothing.

If the disk activity can be picked up by ALMA, but not by Keck, "it makes us think that there is a particular process going on that is not yet fully understood," says Ciurlo. More observations with ALMA and with the Event Horizon telescope could help solve the mystery.