科学美国人60秒:Dark Matter Shell Saved Wannabe Galaxy

Dark matter is that mysterious stuff that apparently accounts for some quarter of the mass and energy in the universe. And for one small wannabe-galaxy, dark matter may have been a lifesaver.

It’s called the Smith Cloud. Astronomers think it’s a failed dwarf galaxy that lacked the requisite mass to produce stars. Many millions of years ago the cloud seems to have collided with the Milky Way and passed through the disk of our galaxy. A new computer simulation finds that the cloud should have been ripped to shreds. That it survived is evidence that it’s protected—by a shell of dark matter.

The report will appear in the journal Monthly Notices of the Royal Astronomical Society. [Matthew Nichols et al, The Smith Cloud and its dark matter halo: Survival of a Galactic disc passage]

That dark matter shell could have insulated the Smith Cloud from the Milky Way’s gravitational forces, which otherwise should have torn Smith apart. 

As if its first fender-bender with the Milky Way wasn’t enough, the Smith Cloud is coming back for more. It’s on course to slam into our galaxy again—in about 30 million years. Perhaps future astronomers will note whether its dark matter shroud once again saves the cloud.

—Clara Moskowitz