Some Galaxies Are Made Almost Entirely of Dark Matter

by Fraser Cain on February 26, 2007

Dark matter distribution. Image credit: Max-Planck Institute for AstrophysicsWhen we think of a galaxy, we think of our own Milky Way or perhaps Andromeda; a majestic spiral containing hundreds of billions of stars. Or maybe we think of an irregular galaxy, not so majestic-looking, but still made of regular stuff, like stars, planets… people.

But new research shows that there are galaxies out there which are almost completely comprised of dark matter. They’re called dwarf spheroidals, and they only contain a few stars and almost no gas. Instead, they’ve got an overwhelming amount of dark matter, whose gravity compacts what few stars it has into a roughly spherical shape. And because they don’t have many stars, they’re hard to see, even when they’re nearby.

An international team of researchers has developed a simulation to explain how galaxies like this could form. They used supercomputers to calculate how galaxies interact. When a smaller galaxy collides with a much larger galaxy, friction causes the gas to slow down and be stripped out a galaxy, while the stars and dark matter continue on.

Without this gas, the galaxy can’t continue making stars. It’s only got the stars that had formed before the collision. A massive galaxy can also strip away stars and material through a process called “tidal shocking”. Between these two effects, you can end up with a galaxy devoid of regular matter – all that’s left is dark matter.

Original Source: Stanford News Release

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Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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