Astronomers have long treated the Milky Way as a kind of cosmic yardstick: if something is true here, maybe it is true everywhere.
However, a new study of our nearest large galactic neighbor, M31, also known as the Andromeda Galaxy, argues that this assumption can be deeply misleading, especially in terms of how this cosmic giant kills off smaller neighboring galaxies.
Published in the Monthly Notices of the Royal Astronomical Society and available on the arXiv preprint server, the work focuses on dwarf galaxies. These are the small, faint systems that swarm around big galaxies like the Milky Way and Andromeda. These dwarfs are crucial testbeds for understanding how galaxies grow, how stars form, and how they stop forming.
Many of the dwarf galaxies around the Milky Way are “quenched”: they once formed stars but are now essentially dead, having lost or used up their gas. For years, a popular theory has been that big galaxies like ours shut these dwarfs down by stripping away their gas, heating it, or cutting off fresh supplies as they fall into the larger galaxy’s gravity well. However, the new study shows that, at least around Andromeda, that story is only part of the picture.
Analyzing Dwarf Galaxies
The team looked at 39 dwarf galaxies orbiting Andromeda and asked two basic questions: When did each dwarf fall into Andromeda’s sphere of influence, and when did it stop forming stars? While it is impossible to observe billions of years of motion, the team used dwarf galaxy data from the European Space Agency’s Gaia mission and large cosmological computer simulations that track the growth of thousands of galaxies over the age of the universe.
For each real dwarf, they searched the simulations for many “look‑alike” dwarfs with similar positions, speeds, and masses. Those simulated twins carry full orbital histories, allowing the researchers to estimate, probabilistically, when the real dwarfs likely first fell toward Andromeda and when they made their first close pass.
Artistic representation of the ESA’s Gaia spacecraft detecting the Ophion star family in April of this year (Credit: ESA/GAIA/DPAC).
“We present predictions for proper motions, infall times and times of first pericentric passage for 39 of M31’s satellite galaxies,” the authors write in the study. “We use these constraints on the satellites’ orbital histories in conjunction with their published star formation histories to investigate the dominant environmental mechanisms for quenching satellites of M31-like hosts.”
They then compared those inferred timelines with existing reconstructions of when each dwarf stopped forming stars, based on detailed studies of their stellar populations. In simple terms, they asked: Did these galaxies die before or after they came under Andromeda’s control? The answer turned out to depend strongly on the size of the dwarf galaxies.
The more massive dwarfs (those with relatively deep gravity wells) tend to keep forming stars for a long time and often quench only after spending some time close to Andromeda. That points to Andromeda’s own environment as the likely killer, through processes like ram‑pressure stripping (gas being blown out as a galaxy plows through hot surrounding material) or tidal effects near close passages.
But the smaller, fainter dwarfs tell a very different story. For most of them, the study finds that star formation shut down well before they entered Andromeda’s halo. In other words, Andromeda cannot be blamed for their deaths—something else got to them first.
Loss Before Capture
The authors explain that many of these tiny galaxies were likely “pre‑processed” in smaller groups or in dense filaments of matter, losing their gas and shutting down before they were ever captured by a big galaxy. For only the very faintest few, the timing is early enough that the flood of radiation from the young universe, known as cosmic reionization, could have played a key role in shutting off star formation.
So why do astronomers care about a bunch of dead dwarf galaxies?
First, the research indicates that the environment of a large galaxy is not the only, or even the main, player in killing off small galaxies. The evolution of a dwarf galaxy as it moves about the universe, be it in small groups, passing through dense gas, or being born light enough to be vulnerable to early radiation, can be just as important as its final plunge toward a massive neighbor. In other words, the development of that galaxy is just as important as its final moments getting devoured by its giant neighbor. The old simple “one‑size‑fits‑all” picture of how a galaxy dies may force theorists to build more nuanced models that track not only where a galaxy ends up, but where it has been.
Second, the study highlights that the Milky Way is not a perfect stand‑in for the rest of the universe. When the authors compare Andromeda’s dwarfs to those around the Milky Way, they find that our own satellites are unusually old and quenched very early, often soon after falling in. Around Andromeda, by contrast, there is a much broader mix. Some dwarfs that quenched before arriving, some that died near their first close pass, and some that are still forming stars today. Other independent work has also suggested that the Milky Way assembled its mass unusually early compared with similar‑sized galaxies.
In all, these lines of evidence suggest that Andromeda’s satellite system may actually be more representative of what is typical in the universe than our own backyard.
Studying Galactic ‘Cemetaries’
For years, astronomers have used the Milky Way’s dwarf galaxies to test ideas about dark matter, star formation, and feedback from exploding stars. If our satellite population is unusual, then theories tuned only to match the Milky Way risk being biased. Studies like this one argue for a broader, more comparative approach: use Andromeda and other nearby systems as equally important laboratories, and demand that models simultaneously explain their more varied satellite histories.
To understand the universe, we need to study not just our galactic cemetery but also those of our neighbors.
“With probable orbital histories now available for satellite galaxies across the Local Group, we have a wider sample than ever for examining how satellites interact with their hosts,” the authors explain.
“The properties of M31’s satellites reflect the fact that environmental effects – rampressure, tidal stripping or the cessation of gas accretion – are reliable quenchers of low mass satellite galaxies in the Universe,” they conclude.
The study, “The lives and deaths of faint satellite galaxies around M31,” is currently available on the preprint server arxiv.org.
MJ Banias covers space, security, and technology with The Debrief. You can email him at mj@thedebrief.org or follow him on Twitter @mjbanias.