Dark-Matter Jacket Allows Failed Dwarf Galaxy to Survive Galactic Collision

Astronomers believe that a high-velocity hydrogen cloud hurtling toward the Milky Way is wrapped in a dark-matter jacket that has allowed it to survive its past galactic collision with the disk of our galaxy.

This high-velocity cloud (HVC) known as the Smith Cloud could also be, upon further observation by the National Science Foundation's Robert C. Byrd Green Bank Telescope (GBT), a failed dwarf galaxy - an object that has all the right materials to form a true galaxy, just not enough to produce stars.

"The Smith Cloud is really one of a kind. It's fast, quite extensive, and close enough to study in detail," lead author Matthew Nichols with the Sauverny Observatory in Switzerland said in a statement. "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."

Previous studies of the Smith Cloud revealed that it first passed through our Galaxy many millions of years ago. Traveling about 150 miles per second, the 8,000-light-year-away cloud is predicted to impact with our galaxy again, but not for about another 30 million years.

By reexamining the cloud, astronomers now believe that the Smith Cloud contains and is actually wrapped in a substantial "halo" of dark matter - the gravitationally significant yet invisible element that makes up roughly 80 percent of all the matter in the Universe.

Most HVC have a common origin with the Milky Way, but a rare few are interlopers from farther off in space, and a halo of dark matter makes astronomers believe it is one of these rare exceptions.

"If confirmed to have dark matter this would in effect be a failed galaxy," Nichols said. "Such a discovery would begin to show the lower limit of how small a galaxy could be." The researchers believe this could also improve our understanding of the Milky Way's earliest star formation.