Imma Perfetto
Cosmos science journalist
The Andromeda Galaxy is the nearest major galaxy to ours, the Milky Way. On moonless nights it’s visible to the naked eye, but it takes a powerful space telescope to see that Andromeda isn’t as alone as it might appear.
It is surrounded by a swarm of nearly 3 dozen dwarf galaxies, which circle it like bees around a hive. These “satellite galaxies” have been studied in unprecedented detail in a new paper published in the Astrophysical Journal.
Astronomers used data captured by the Hubble Space Telescope to build a precise 3-dimensional map of their motion. They then reconstructed how these galaxies would have formed stars over billions of years.
The findings show that Andromeda’s satellite galaxies evolved very differently compared to the ones orbiting the Milky Way.
The data indicates that it probably had a more dynamic history than the Milky Way and was affected by a major merger with another large galaxy a few billion years ago.
“Everything scattered in the Andromeda system is very asymmetric and perturbed,” says Daniel Weisz of the University of California (UC) at Berkeley in the US.
“It does appear that something significant happened not too long ago.”
Half of the satellite galaxies also seem to be confined to a single plane, orbiting Andromeda in the same direction.
Weisz says that’s weird.
“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.”
According to lead author Alessandro Savino, also from UC Berkeley, the duration for which the satellites continued forming new stars depends on how massive they are and on how close they are to the Andromeda galaxy.
“It is a clear indication of how small-galaxy growth is disturbed by the influence of a massive galaxy like Andromeda,” says Savino.
The data also revealed a unique population of dwarf galaxies which formed most of their stars very early on and then kept forming them out of a reservoir of gas at a very low rate.
“Star formation really continued to much later times, which is not at all what you would expect for these dwarf galaxies,” says Savino.
“This doesn’t appear in computer simulations. No one knows what to make of that so far.”
