Middleweight black hole suspected near Milky Way’s center

The supermassive black hole at the center of the galaxy might have a middleweight neighbor, a new study suggests. If this partner exists, it would be the second most massive black hole known in the Milky Way.

A black hole weighing the same as about 100,000 suns could explain why gas in an interstellar cloud is swirling around at hundreds of kilometers per second, researchers suggest. Tomoharu Oka, an astrophysicist at Keio University in Yokohama, Japan, and colleagues present the findings in the Jan 1. Astrophysical Journal Letters.

Intermediate mass black holes — between 100 and 1 million times as massive as the sun — are highly sought after but hard to find. They might help researchers understand the relationship between black holes with the mass of a few suns and the behemoths that are up to several billion times as massive. Astronomers have yet to turn up definitive evidence for an intermediate black hole, though some blazing sources of X-rays seen in other galaxies are candidates.   

This new candidate and its host cloud sit in the Milky Way’s nucleus, about 25,000 light-years away in the constellation Sagittarius. Just 200 light-years from the pair lies the region’s gargantuan black hole, weighing in at roughly 4 million suns. Oka and colleagues discovered the turbulent cloud using the Nobeyama Radio Observatory in Japan. Searches through old X-ray and infrared images for the source of the turbulence turned up nothing. Given how tiny and massive the culprit must be, the researchers argue that the cloud is passing by an invisible intermediate mass black hole whose gravity is stirring up the gas.

There are a couple of problems with this interpretation, says Cole Miller, an astrophysicist at the University of Maryland in College Park. As a black hole with this much mass plows through an interstellar cloud, it should generate radiation about one billion times as bright as the sun. And given that the black hole would spend only about 10,000 years passing through the cloud — an eyeblink on cosmic timescales — the odds of finding the two together are pretty low. “We would have to be remarkably lucky,” he says.

Oka says he plans to keep an eye on the cloud for any flashes triggered by gas colliding around the putative black hole. If it’s there, “we expect faint X-ray emission or a kind of flare-up,” he says.