“Keep up the good work, if only for a while, if only for the twinkling of a tiny galaxy.” –Wislawa Szymborska
Our Milky Way Galaxy is home to not only our Earth and our Solar System, but hundreds of billions of other stars.
Held together by not only the incredible gravity of all of our stars, but by dark gas and dust far outweighing all the stars, and by trillions of suns worth of dark matter as well, our galaxy represents one of perhaps a hundred billion just like it in our vast Universe.
But these galalxies are massive and bright. What if I wanted to know what was happening at the other end of the spectrum: the smallest, lightest, dimmest objects in the Universe?!
You would think to look at small, satellite galaxies, like Leo A, above. But with millions of stars in even the smallest dwarf galaxies, we can go much smaller.
You might even think to go to a globular cluster, which is even smaller and less massive, with just hundreds of thousands of stars in most of them.
But very recently, we’ve been able to do one better. Take a look at this area of the sky, which houses the smallest “mini-galaxy” in the known Universe.
See it? Of course you don’t. Most of the stars in this image come from our galaxy, and the mini-cluster, gravitationally bound together, is just a small number of faint stars in this image.
But using the Keck’s Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) instrument, they were able to measure how these stars are moving relative to both each other and to the Milky Way galaxy.
They found that these stars — part of a collection of only about one thousand — are all bound together and moving with an average speed of 209 km/s relative to the Milky Way.
If this were just a cluster of 1,000 stars bound together under their own gravity, you’d expect the difference between the fastest and slowest-moving stars to be less than 1 km/s. But astronomers found that the slowest stars were moving at 194 km/s, while the fastest move at 224 km/s, meaning there is a tremendous amount of dark matter!
For just 1,000 stars, there is a total mass in this mini-galaxy of 600,000 Solar Masses, meaning that nearly all of the matter in there is dark! With a total mass hovering right around the minimum-sized structure the Universe can form, this is absolutely amazing! It’s also one of the oldest objects in the Universe, as pretty much all of the stars are very old, metal poor, Population II stars.
Now this is particularly interesting because one of the things we found, perhaps disturbingly, is that dwarf galaxies (like Leo A, close to the top) have much more dark matter compared to the amount of stars they have versus a galaxy like ours. Proponents of dark matter had to face the fact that — if there really was a giant halo of dark matter surrounding every galaxy — the number of stars you formed must be related to the total mass of the galaxy in a non-trivial way.
One of the predictions that this made was that the smaller and smaller your gravitationally bound structure got, you should be forming even fewer stars than you would expect by proportion. With less than 0.03% of the mass of this galaxy in stars (versus about 2% for a galaxy like ours), this prediction is correct!
And so — in perhaps the most unlikely of places — the smallest galaxy ever discovered helps us understand dark matter just a little better! Thanks to Tammy Plotner for her article on this story, which brought it to my attention!