Failed Dwarf Galaxy Wrapped in Dark Matter Survives Deadly Collision with Milky Way

Scientists have spotted an amazing phenomenon within the depths of space. They've found a high-velocity hydrogen cloud hurtling toward the Milky Way that appears to be encased in a shell of dark matter. The new findings could tell researchers a bit more about the nature of this hydrogen cloud.

The cloud in question is known as a high-velocity cloud (HVC) called the Smith Cloud. HVCs are made up primarily of hydrogen gas that is too rarefied to form stars in any detectable amount.  Most HVCs share a common origin with our own galaxy, either as leftover building blocks of galaxy formation or as clumps of material launched from supernovas. Others, though, are interlopers from farther off in space-and the Smith Cloud may very well be one of these rarer cases. Scientists believe that without the protective shell of dark matter, the Smith cloud would have disintegrated long ago when it first collided with the disk of our galaxy.

"The Smith Cloud is really one of a kind," said Matthew Nichols, one of the researchers, in a news release. "It's fast, quite extensive, and close enough to study in detail. It's also a bit of a mystery; an object like this simply shouldn't survive a trip through the Milky Way, but all the evidence points to the fact that it did."

In fact, previous studies have shown that this cloud first passed through our galaxy millions of years ago. Now, astronomers have reexamined and modeled the cloud and have found that it must contain and be wrapped within a "halo" of dark matter.

"Based on the currently predicted orbit, we show that a dark matter free cloud would be unlikely to survive this disk crossing," said Jay Lockman, one of the researchers, in a news release. "While a cloud with dark matter easily survives the passage and produces an object that looks like the Smith Cloud today."

The findings reveal that it's possible that the Smith Cloud could actually be a failed dwarf galaxy. If these findings about dark matter are true, the Smith Cloud could reveal the lower limit of exactly how small a galaxy could be and also show astronomers a little bit more about our own galaxy's evolution.

The findings are published in the journal Monthly Notices of the Royal Astronomical Society.