Dozens of dwarf galaxies swarming around the Andromeda Galaxy like bees have been caught on camera by the Hubble Space Telescope, which took more than a thousand orbits of the Earth to take enough images to get a full family portrait of Andromeda’s brightest satellites.
The 36 dwarf galaxies are varied in their properties, and many exhibit puzzling orbits that conventional theories have so far been unable to explain. The solutions to these mysteries could reveal much about how the Andromeda Galaxy — which is the closest large spiral galaxy to us, sitting about 2.5 million light-years away — formed and developed.
“We do find that there is a lot of diversity that needs to be explained in the Andromeda satellite system,” said Daniel Weisz of the University of California, Berkeley. “The way things come together matters a lot in understanding this galaxy’s history.”
The concept of dwarf galaxies swarming around larger galaxies is expected from models of galaxy formation based on the standard model of cosmology, which depicts a universe filled with what’s known as “cold dark matter.” In this scenario, galaxies are assembled within a structure of dark matter, the gravity of which draws in small clumps of normal matter — aka, the dwarf galaxies — and merges them together. This is known as hierarchical formation. The satellite galaxies that we see today are the remnants of this construction phase, which took place in the early universe, beginning over 13 billion years ago.
As such, the family of dwarf galaxies buzzing around a larger galaxy, such as the Andromeda Galaxy, can tell us a great deal about the larger galaxy’s formation history.
This new Hubble Space Telescope image represents just half of a planned project to study the motions of Andromeda’s dwarfs. In five years time, Hubble or the James Webb Space Telescope will collect more data to produce another image like this; by comparing the two images, astronomers will be able to see how the dwarf galaxies have moved. Then, by backtracking, they’ll be able to sort of rewind time and run the dwarf galaxies backwards to see where the realms came from in the first place, billions of years ago, and understand how this impacted their relationship with Andromeda.
Some of Andromeda’s dwarf galaxies are particularly puzzling, though. Dwarf galaxies around the Milky Way, for example, either stopped forming stars billions of years ago, or have only recently begun forming them again after they entered our galaxy’s sphere of influence, where gravitational tidal forces stirred up pockets of gas to reignite the process of star birth.
However, it seems that some of Andromeda’s dwarf galaxies never stopped forming stars, continuing to do so at a slow rate throughout cosmic history.
“Star formation really continued to much later times, which is not at all what you would expect for these dwarf galaxies,” Alessandro Savino of the University of California, Berkeley, who led the study into Andromeda’s satellites, said in a statement. “This doesn’t appear in computer simulations. No one knows what to make of that so far.”
One clue comes from how close these dwarf galaxies are to their parent galaxy. “It is a clear indication of how small-galaxy growth is disturbed by the influence of a massive galaxy like Andromeda,” said Savino.
Another peculiarity is how the orbits of some of the dwarfs are arranged, with half of them orbiting around the Andromeda galaxy in the same plane, and all in the same direction.
“That’s weird,” said Weisz. According to the standard model, the dwarf galaxies should approach the dark matter halo — within which a large galaxy is growing from all angles — and therefore swarm around it on random orbits. For the galaxies to all be in the same plane and move in the same direction seems like too much of a coincidence.
“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,” said Weisz.
Although the standard model of galaxy formation involving dark matter haloes struggles to explain why half of Andromeda’s dwarf satellite galaxies are in this plane, the phenomenon is actually a prediction of an alternative theory of gravity, Modified Newtonian Dynamics or MOND, which seeks to replace dark matter by tweaking the laws of gravity at low accelerations.
In the MOND scenario, the Milky Way and the Andromeda galaxies would have experienced a close encounter to one another about 8 billion years ago. The proximity of the Andromeda Galaxy to the Milky Way would have seen material ripped out of our galaxy to settle in the same orbital plane around the Andromeda galaxy and form the dwarf galaxies there, a bit like moons forming around a planet. However, another team of astronomers has previously disputed this, saying that the plane is just a chance alignment. Still, the new Hubble observations may suggest otherwise.
So, while it is still possible that a solution could be found under the standard model paradigm, the confirmation that this plane of satellite galaxies exists will be seen as a significant victory in the eyes of MOND theorists.
Either way, the observations emphasize the fact that our Milky Way galaxy and the Andromeda galaxy have experienced significantly different histories. Studies of our own galaxy’s growth imply it has been a rather placid place with no major mergers with medium or large galaxies in the past 10 billion years at least, whereas the Andromeda galaxy shows signs of having experienced a significant merger with another galaxy in the past few billion years. This is supported by other recent Hubble observations that imaged 100 million stars in the Andromeda galaxy. This merger could have contributed to making the environment around Andromeda more dynamic, as well as bulking up Andromeda itself (it is twice as massive as our Milky Way, and has more satellite galaxies).
This calls into question how wise it is to use our own galaxy as a template for other galaxies.
“There’s always a tendency to use what we understand in our own galaxy to extrapolate more generally to the other galaxies in the universe,” said Weisz.
Evidence is growing that galaxies can be quite diverse, with different properties and histories, and how their family of satellites have developed can be a sign that something is different. “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,” concluded Weisz.
It seems that in the galaxy zoo, the lions and tigers can differ more wildly than we imagined.
The new findings were published on Jan. 28 in The Astrophysical Journal.
