Astronomers Image Cold Gas Ring Around Our Galaxy’s Central Black Hole

Scientists recently produced the first images of a giant black hole in a distant galaxy, but there’s a supermassive black hole much closer to home. In the center of our own galaxy lurks an invisible monster, the gravity of which may help hold the Milky Way together. We can’t see this black hole, but observations have detected some of its effects like a sphere of superheated gas. A new study has now revealed the other side of the coin, a ring of comparatively cool gas around the black hole.

This black hole, known as Sagittarius A* (pronounced “Sagittarius A Star”) lies about 26,000 light years from Earth. It’s difficult to observe because we have to look through the disk of the Milky Way, but observations have shown that this region of space has densely packed stars, nebulae, and clouds of hot and cold gas. The gas should form a rotating accretion disk that extends several tenths of a light-year from the black hole’s event horizon.

Previously, all we’ve been able to see is the hot portion of that gas via millimeter wave observations, which gives an incomplete picture of the black hole’s effects on nearby space. All scientists have been able to say before now is that there was a cold gas component, but now we’ve got a true picture of it.

Of course, “cold” gas is a relative term here. The hot gasses around Sagittarius A* are around 18 million degrees Fahrenheit (10 million degrees Celsius), which is two-thirds the temperature of the sun’s core. This gas emits X-rays, one of the hallmark signs of a black hole. The cold hydrogen gas, by comparison, is only about 18,000 degrees Fahrenheit (10,000 degrees Celsius). Researchers used the Atacama Large Millimeter/submillimeter Array (ALMA) to scan for the faint radio frequency signal from this ring of cooler gas.

The resulting image (top) shows the flow of cold gas around Sagittarius A*. Mapping the Doppler shift in the spectrum as the gas moves toward and away from us revealed a ring-like structure. The red part is moving away from Earth and the blue part is moving toward Earth. The team says this ring of gas extends to just a hundredth of a light year from the event horizon (about 1,000 times the distance from Earth to the sun).

This data could provide new insights into how black holes consume nearby matter. Hopefully, that’s something we’ll never have to worry about on Earth, but it may help us understand the happenings in other parts of the universe.

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