When talking about space, people often gravitate to the biggest, the brightest, and the closest to us. But sometimes it’s just as fascinating to learn about the other end: the faint and seemingly insignificant. Ultra-diffuse galaxies are in the latter camp. These dim, wispy galaxies have caught the attention of astronomers, who have sought to find out how they formed.
WHY THEY’RE INTERESTING?
Ultra-diffuse galaxies are fairly large, sometimes stretching to the size of our own Milky Way galaxy. But they contain the same number of stars as your average dwarf galaxy; that is, as little as one one-thousandth the number in the Milky Way. That makes them incredibly dim and hard for astronomers to spot, which is why nearly all of the ultra-diffuse galaxies we’ve discovered so far have been in clusters of other galaxies. The telescopes were already looking at the clusters and spotted those galaxies by happenstance.
But how does a galaxy with so few stars get to be so big? Since the discovery of the first ultra-diffuse galaxy in 2015, astronomers have been split. Some thought that they were ordinary spiral galaxies that contained an unusually large amount of dark matter, while others thought they were just dwarf galaxies that had spread out over a larger area. In November 2016, scientists solved the mystery. By using computer simulations, researchers in Copenhagen and Abu Dhabi were able to watch the formation of nearly 100 virtual galaxies from characteristics they had observed in real ones. What they saw was that young ultra-diffuse galaxies start out like dwarf galaxies but contain a large number of supernovae, whose vast explosions blow other stars and dark matter outward until the whole galaxy is extra large and extra faint. Like a housecat that inexplicably takes up half of a queen-size bed, ultra-diffuse galaxies are dwarf galaxies that have spread out and gotten comfortable.
WHY THEY’RE IMPORTANT?
Dwarf galaxies are the most numerous type of galaxy in the universe. The fact that ultra-diffuse galaxies are dwarf galaxies’ larger sisters means that there are probably many, many more of them out there that we haven’t spotted. Add that to the list of things we never knew we didn’t know.