Surrounding the Andromeda galaxy, three dozen tiny galaxies aren't behaving the way scientists expected.
NASA's Hubble Space Telescope has trained on Andromeda for a deep dive into how its orbiting satellite galaxies formed and changed over time. What they found revealed a population of dwarf galaxies that are quite unlike the ones circling the Milky Way.
Some 2.5 million light-years away, these space neighborhoods formed the bulk of their stars long ago. But rather than halting production, as computer simulations would suggest, they continued slowly making new stars out of a stockpile of gas.
"Star formation really continued to much later times, which is not at all what you would expect for these dwarf galaxies," said Alessandro Savino, an astronomer at UC Berkeley, in a statement. "No one knows what to make of that so far."
In the past, scientists primarily observed dwarf galaxies near the Milky Way, but they were never sure whether those were representative of others in the universe. That's why they pointed Hubble, which orbits Earth, at the closest large galaxy, which has its own bevy of satellite galaxies.
The study, published in The Astrophysical Journal, is based on observations from over 1,000 telescope orbits. The sweeping science campaign allowed astronomers to build a detailed 3D map of Andromeda's 36 dwarf galaxies and reconstruct how they made new stars over the 13.8 billion years since the Big Bang. The images have provided a unique bird' s-eye view of Andromeda and its environment.
In addition to the dwarf galaxies' prolonged star-bearing years, scientists were surprised to find that half were sitting on the same plane and moving in the same direction. However, mergers and collisions usually result in objects traveling in inconsistent directions.
"That's weird," said Daniel Weisz, the principal investigator at UC Berkeley, in a statement. "It was actually a total surprise to find the satellites in that configuration, and we still don't fully understand why they appear that way."
Astronomers have learned that galaxies tend to start out small and grow larger by collecting gas and merging with other galaxies. But most dwarf galaxies that made stars before the so-called Epoch of Reionization never got back to business after. Reionization was an era of major transition that occurred more than 13 billion years ago. It was when the baby universe transformed from a neutral state to one filled with free electrons and protons.
The above animation gives a fly-around view of the Andromeda galaxy and its surrounding dwarf galaxies, based on Hubble data.
Because most of the tiny galaxies turned off their star-making activity in the first few billion years of the universe, many scientists have thought reionization was the reason. However, some researchers are calling that idea into question.
Astronomers suspect Andromeda had a major collision with another galaxy relatively recently, perhaps 2 to 5 billion years ago. The Milky Way, on the other hand, probably hasn't had a run-in with another galaxy for 8 to 10 billion years. Andromeda's collision — and its more massive scale — could explain the galaxy's exotic and diverse satellite system.
The study has only deepened the team's speculation that dwarf galaxies aren't all like the ones close to home. They can have a wide range of fates, the researchers posit, and it'll take more observations to ascertain why.
"Everything scattered in the Andromeda system is very asymmetric and perturbed. It does appear that something significant happened not too long ago," Weisz said. "Our work has shown that low-mass galaxies in other ecosystems have followed different evolutionary paths than what we know from the Milky Way satellite galaxies."
