Sizing Up a Galaxy’s “Exit Door”

The point of no return: in astronomy, it’s known as a black hole—a region in space where the pull of gravity is so strong that nothing can escape.

An international team led by researchers at MIT has measured the closest point at which matter can approach a black hole at the center of a distant galaxy before being pulled in.

The scientists linked radio dishes in Hawaii, Arizona, and California to create a telescope array called the Event Horizon Telescope (EHT), whose resolution is 2,000 times finer than that of the Hubble Space Telescope. These radio dishes were trained on M87, a galaxy some 50 million light-years from the Milky Way. M87 harbors a black hole six billion times more massive than our sun. Using the array, the team observed the glow of matter near the edge of this black hole, signifying the event horizon—the point at which not even light can escape.

“Once objects fall through the event horizon, they’re lost forever,” says Shep Doeleman, assistant director at MIT’s Haystack Observatory. “It’s an exit door from our universe. You walk through that door, you’re not coming back.”

Supermassive black holes exert the most powerful gravitational forces in the universe, as predicted by Albert Einstein’s theory of relativity. In a black hole, according to Doeleman, “gravity completely goes haywire and crushes an enormous mass into an incredibly close space.” However, not all the surrounding material can cross the event horizon to squeeze in. The result is a “cosmic traffic jam” in which gas and dust build up into a flat pancake of matter known as an accretion disc, which orbits the black hole, feeding it a steady diet of superheated material.

Using a technique called very long baseline interferometry, which combines data collected by radio dishes located thousands of miles apart, the team was able to make the most detailed observations ever obtained of the black hole at the center of M87. The diameter they measured for the innermost stable orbit of the accretion disc is 2.25 times the diameter of the event horizon, or the size of the black hole itself. Taking into account some predictions from ­Einstein’s theory of general relativity, the researchers also used this measurement to infer that the black hole is spinning in the same direction as the accretion disc.

“People have converged on that scenario based on very detailed simulations on supercomputers,” Doeleman says. “But all those theoretical models and simulations have been blissfully unencumbered by real observations. So this marks a real turning point … that will start to constrain all of these theories.”

Doeleman has published the results in Science.