The working definition of a galaxy is a huge collection of stars whose motions we do not understand. It was therefore a surprise when a galaxy was recently discovered whose motions we do understand. This new type of galaxy is called an Ultra Diffuse Galaxy (UDG).
Astronomers usually ascertain the mass of a galaxy by observing the speeds of the stars and star clusters that orbit it. There is a straightforward formula which relates the speed of the stars with the mass of the galaxy.
The Sun and the Earth, for example, move at a speed of 475,000 miles per hour about the center of the Milky Way. From this fact we work out that the mass of the Galaxy is about 300 billion times the mass of the Sun. But what is this mass anyway?
Well, embarrassingly we cannot even see the majority of the mass in the Milky Way. Put another way, there is some form of material, called dark matter, which dictates how the Sun and another 300 billion stars will move yet which we cannot identify. Now we can see why the working definition of a galaxy is a collection of stars whose motions we do not understand.
An interesting new chapter arises with the discovery of the UDG. Like a house built out of glass so that you can see all its contents, this galaxy also leaves no mysteries about what is inside of it. By studying the speeds of old groups of stars called globular clusters we measure a mass that is exactly what we expect if we just sum up the mass of its huge collection stars.
To add to this developing story, globular clusters are usually associated with massive, dark-matter dominated galaxies. We are driven to ask a different question: why do we understand the motion of the stars of an UDG? Where did its dark matter go, or are there some galaxies for which the dark matter was never there?
