Earth is located in the Milky Way, a galaxy estimated to hold between 100 billion and 400 billion stars. But astronomers are beginning to find hundreds of galaxies with hardly any stars at all, and figuring out why they are so dark may help them understand how galaxies form.
In 2015, astronomers found the first batch of these “ghost” galaxies, 47 of them roughly the size of the Milky Way. “If you took the Milky Way but threw away about 99 percent of the stars, that’s what you’d get,” said Roberto Abraham, in a recent interview with Science News. Abraham, an astrophysicist at the University of Toronto, noted, “We’ve gone from none to suddenly over a thousand.”
Astronomers have been struggling to determine what unseen matter is keeping these “dark” galaxies from the gravity of other galaxies pulling them apart. “We speculate that these ‘ultra-diffuse galaxies’ may have lost their gas supply at early times, possibly resulting in very high dark matter fractions,” Abraham and other astronomers noted in their initial study which discovered these types of galaxies.
The technology to find “ultra-diffuse galaxies” has only been recently available. The Dragonfly telescope doesn’t use mirrors, and its lenses are designed to minimize the reflection of light that make these faint galaxies undetectable by other telescopes. Its use on previously studied galaxy clusters have led to the discovery of hundreds of ultra-diffuse galaxies.
How these galaxies formed, and what keeps them intact has remained an enigma for astronomers. If a galaxy is in the process of forming stars and happens to get pulled into a galaxy cluster, the resulting headwind can strip that galaxy of the gas it needs to form stars, according to one proposed theory.
Other astronomers have hypothesized that some galaxies rotate rapidly enough for its stars and gas to spread far out, minimizing its ability to form large numbers of stars.
A simulation recently conducted by the Numerical Investigation of a Hundred Astronomical Objects found that these ultra-diffuse galaxies develop through internal dynamics during formation that expand an initial dwarf galaxy into a large, but sparsely-populated one.
“Dwarf galaxies undergo repeated episodes of star formation, at the end of which some stars explode as supernovas, releasing energy to the surrounding gas, which is brought to high temperatures and then very quickly expelled from the galaxy center,” Arianna di Cintio, a researcher at University of Copenhagen’s DARK Cosmology Center and lead author on the simulation, told an Italian media outlet. Di Cintio noted this quick expulsion of gas occurs in dwarf galaxies in the early stages of formation. It could cause the galaxy to swell from a dwarf to a large sized galaxy but with very few stars to fill it. “Our work shows that the formation mechanism of ultra-diffuse galaxies are internal and not directly connected to a cluster environment: We therefore expect to find ultra diffuse galaxies everywhere, even ‘isolated,’ that is far from clusters.”
Though most of the galaxies have been discovered near clusters as that is where telescopes tend to focus their sights on, Di Cintio said her team’s research will now focus on discovering isolated ultra diffuse galaxies. She anticipates finding thousands of galaxies astronomers were previously unaware existed, even in regions of space that have been studied for decades.