Like most galaxies, the center of the Milky Way contains a supermassive black hole. And now astronomers have discovered a gas cloud that made a tight orbit around our black hole, called Sagittarius A*, over 10 years ago. They’ve dubbed the object G1. 

In 2011, researchers from Max Planck Institute for Extraterrestrial Physics detected a gas cloud they called G2, but not everyone agrees that the object was a gas cloud. Some say G2 is a star surrounded by a layer of dust and gas, Space.com explains: When G2 made its closest approach to the black hole this summer, it wasn’t torn apart, suggesting that it’s a solid body. However, according to the Max Planck team, a gas cloud could have stayed intact. They argue that G2 was once part of a larger gas streamer that broke up into smaller gas clouds, which all follow the same path -- like pearls on a string. Identifying additional gas clouds following G2’s path could strongly support their claim. 

In this new work, a team led by Max Planck’s Oliver Pfuhl and Stefan Gillessen tracked the object’s ongoing disruption by the black hole’s gravitational field using deep infrared observations at the Very Large Telescope. They were surprised to find that a faint and blurry object had already passed the point of closest approach to the black hole in 2001. “Already a decade ago, another gas cloud -- which we now call G1 -- has been observed in the central region of our galaxy,” Gillessen says in a news release. “We explored the connection between G1 and G2 and find an astonishing similarity in both orbits.” That similarity suggests that G1 is about 13 years ahead of G2.

So, the scientists fed this information into a model for a combined orbit. “Our basic idea is that G1 and G2 might be clumps of the same gas streamer,” Pfuhl explains. “In this case, we should be able to simultaneously fit both data sets and, indeed, our model captures the G1 and G2 orbits remarkably well.”

Above are the gas clouds G1 (blue) and G2 (red) at the center of the Milky Way. The spherical cloud has been stretched by the black hole’s gravitational field. The red side approaches the black hole (indicated with a cross) at velocities of a few thousand kilometers per second; the blue part has already passed its closest distance to the black hole and is moving away again. The solid line shows the best-fit common orbit for the two gas clouds, and the dashed line shows the orbit of S2, the star with the best known orbit. 

A likely source for both G1 and G2, they think, could be clumps in the wind of one of the massive disk stars. The new findings will be published in the Astrophysical Journal, and it appears online at arXiv now. 

Images: NASA (top), Max Planck Institute for Extraterrestrial Physics (middle)