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RandomFactor [soylentnews.org] writes:

In a presentation [aps.org] given on April 15th at the American Physical Society in Denver, Researcher Ana Bonaca of the Harvard-Smithsonian Center for Astrophysics presented evidence for a "dark impactor" tearing through our galaxy's longest stellar stream [livescience.com] - GD-1.

Stellar streams are lines of stars moving together across galaxies, often originating in smaller blobs of stars that collided with the galaxy in question. The stars in GD-1, remnants of a "globular cluster" that plunged into the Milky Way a long time ago, are stretched out in a long line across our sky.

Under normal conditions, the stream should be more or less a single line, stretched out by our galaxy's gravity, she said in her presentation. Astronomers would expect a single gap in the stream, at the point where the original globular cluster was before its stars drifted away in two directions. But Bonaca showed that GD-1 has a second gap. And that gap has a ragged edge — a region Bonaca called GD-1's "spur" — as if something huge plunged through the stream not long ago, dragging stars in its wake with its enormous gravity. GD-1, it seems, was hit with that unseen bullet

The impactor doesn't match the path of any luminous object according to Bonica. Additionally it is far more massive than a star, approximately a million times more massive than the sun, and between 10 and 20 parsecs across.

That leaves limited possibilities - possibly a black hole, but this would be a black hole on-par with the supermassive black holes found at the center of galaxies. Additionally

we'd expect to see some sign of it, like flares or radiation from its accretion disk. And most large galaxies seem to have just a single supermassive black hole at their center.

This leaves an intriguing possibility. A dark matter object or structure.

With no giant, bright objects visible zipping away from GD-1, and no evidence for a hidden, second supermassive black hole in our galaxy, the only obvious option left is a big clump of dark matter. That doesn't mean the object is definitely, 100%, absolutely made of dark matter, Bonaca said.

The findings are based on data obtained from the ESA Gaia mission. [space.com]

Bonica's results were well received but have not yet been published in a peer reviewed journal.

Original Submission