Andromeda, our closest galactic neighbor, has swallowed multiple smaller galaxies over its 10-billion-year lifespan. Our own galaxy, the Milky Way, is destined to be Andromeda’s next and most calorific meal once the two galaxies collide in about four billion years.
To better understand Andromeda’s past diet—and to anticipate what might be in store once the Milky Way merges with it—scientists studied the complex gas halo surrounding the galaxy in unprecedented detail in a new study published on Wednesday in Nature.
Co-led by Dougal Mackey, a research fellow and astronomer at the Australian National University, researchers captured a panorama of Andromeda spanning hundreds of thousands of light years.
The results “offer an insight into the assembly history of our nearest large neighbor,” Mackey’s team said in the study. The observations revealed that Andromeda has undergone at least two major feeding episodes—one within the past few billion years, and another during a much more ancient epoch, when it was still a young galaxy.
Mackey and his colleagues probed the digested remnants of these galactic binges with the Pan-Andromeda Archaeological Survey (PAndAS) operated by the Canada–France–Hawaii Telescope on the summit of Mauna Kea, Hawai’i.
As the name implies, PAndAS focuses on reconstructing the origins, evolution, and current structure of Andromeda, making it part of a special astronomical subfield called “galactic archeology.”
“In all likelihood, the collision will be highly disruptive for both the Milky Way and Andromeda"
Though the Milky Way is also a galaxy eater, Andromeda seems to have been more ravenous in the past. Its feeding frenzies are memorialized by two groups of globular clusters, which are dense populations of stars, orbiting Andromeda at perpendicular angles from each other.
“The globular clusters that we measured were probably not formed when smaller galaxies were shredded by Andromeda,” Mackey said in an email. “The most likely scenario, based on the properties of the clusters, is that they were formed in the smaller galaxies, and when those galaxies were later destroyed by Andromeda, the clusters were deposited into Andromeda’s halo.”
The clusters from its most recent feeding event, which the team called the GC-Sub group, orbits Andromeda at an angle close to the galactic disk. In contrast, the clusters from the more ancient galactic meal, called the GC-Non group, arch around the galaxy at a right angle to the disk. This suggests that the two meals were coming from different directions when they were captured and devoured by Andromeda, the team said.
Mackey and his colleagues are not yet sure if Andromeda ate one large galaxy during each of these binges, or if the clusters indicate that several small galaxies were consumed.
Previous studies have suggested that M32, a small satellite galaxy of Andromeda, may be the stripped core of a much larger hypothetical galaxy called M32p. M32p may have been a full quarter of the mass of Andromeda, but much of its bulk was cannibalized by Andromeda about two billion years ago, so the hypothesis goes.
Mackey noted that his team’s observations “are broadly consistent” with this possible scenario, but that there’s currently “insufficient information about the motion of M32 to reliably link the events.” Future observations, including with the Hubble Space Telescope, may yield more insights into whether the recent feeding event featured cosmic tapas or a full galactic entree.
It will be much more difficult to figure out what Andromeda was eating during the older binge, which must have occurred billions of years before the recent one.
“We can no longer see the streams left over from the destruction of whatever system or systems were destroyed,” Mackey said of the ancient feeding epoch. “The stars are there but their spatial positions have long since spread out to form a relatively smooth overall distribution.”
To pursue that mystery, scientists will have to wait until next-generation observatories such as the European Extremely Large Telescope (ELT) are operational. These facilities could also provide precision observations of many nearby galaxies and their mergers, which would shed light on the pasts of both Andromeda and the Milky Way—and their shared galactic future.
“The Milky Way and Andromeda have roughly the same mass,” Mackey explained. “This means that the future collision between the Milky Way and Andromeda won’t proceed in the same way as for the smaller galaxies Andromeda has eaten in the past.”
“In all likelihood, the collision will be highly disruptive for both the Milky Way and Andromeda, leading to the destruction of their spiral disks and the formation of a much more elliptically-shaped merged system,” he said.
Fortunately, scientists predict that this collision will have very limited impacts on individual star systems like our own, so there’s no need to fear the coming era of Milkdromeda. Indeed, assuming the planet still bears life in four billion years, those future Earthlings will be treated to the most brilliant star-studded skies ever seen from our world.