Andromeda’s Dwarf Galaxies Reveal Unique Star Formation Histories

The Andromeda Galaxy, our nearest large neighbour, has 36 identified dwarf galaxies. The Hubble telescope took images of Andromeda and its dwarfs during more than 1,000 orbits, creating a precise 3D map. Astronomers used these observations to reconstruct the dwarf galaxies’ star formation histories.

The results show that their environment plays a critical role in their star formation and their quenching.

When galaxies are quenched, they no longer form stars. It happens because the supply of star-forming gas is diminished or somehow made unavailable. This typically happens because of black hole feedback or when a galaxy moves through a dense galaxy cluster, and its gas is stripped away.

However, the dwarf galaxies around Andromeda (M31) seem to follow an unusual pattern of star formation and quenching. New research shows that the rambunctious environment around M31 is responsible.

The research is “The Hubble Space Telescope Survey of M31 Satellite Galaxies. IV. Survey Overview and Lifetime Star Formation Histories,” published in The Astrophysical Journal. Alessandro Savino from the Department of Astronomy at UC Berkeley is the lead author.

Astronomers aren’t certain how many dwarf galaxies the Milky Way has, but it looks like Andromeda, with its dozens of dwarf galaxies, has had a more active history of mergers and absorptions. M 31 may have merged with another massive galaxy a few billion years ago, and its abundant dwarf galaxies could be from its eventful past and its sheer mass.

“Our knowledge of low-mass galaxy formation has long been anchored by Milky Way (MW) satellite galaxies,” the authors write. “It remains unclear if the insights learned from MW satellites, and their particular formation pathways, are applicable to other satellite systems and low-mass galaxies in general.”

“There’s always been concerns about whether what we are learning in the Milky Way applies more broadly to other galaxies.”

Daniel Weisz, UC Berkeley.

Studying dwarf galaxies is challenging. We’re inside the Milky Way, which makes observing its outskirts difficult. Dwarf galaxies are also dim, adding to their detection difficulty. Detecting them in distant galaxies is likewise difficult. Comparing the MW low-mass dwarf galaxies with those in other galaxies means contending with multiple layers of difficulty. Fortunately, the Andromeda galaxy is wide open to observations.

This large photomosaic of Andromeda is from the Hubble. It's the largest one ever assembled from NASA/ESA Hubble Space Telescope observations. Image Credit: NASA, ESA, B. Williams (University of Washington) — This large photomosaic of Andromeda is from the Hubble. It’s the largest one ever assembled from NASA/ESA Hubble Space Telescope observations. Click on the image to access the full-size version. Image Credit: NASA, ESA, B. Williams (University of Washington)

“From >1000 orbits of HST imaging, we present deep homogeneous resolved star colour-magnitude diagrams that reach the oldest main-sequence turnoff and uniformly measured star formation histories (SFHs) of 36 dwarf galaxies associated with the M31 halo,” the authors write. They did the same for 10 additional fields in M31, M33, and the Giant Stellar Stream. M33 is the Triangulum Galaxy, the third largest member of the Local Group after M31 and the Milky Way. M33 is also one of M31’s satellites. The Giant Stellar Stream is a long ribbon of stars that are the remnants of a galaxy absorbed by M31.

For context, this image shows some of the main features around Andromeda, including the Giant Stellar Stream, M32, and NGC 205, another of Andromeda's dwarf galaxies. Image Credit: Ferguson et al. 2000 — For context, this image shows some of the main features around Andromeda, including the Giant Stellar Stream, M32, and NGC 205, another of Andromeda’s dwarf galaxies. Image Credit: Ferguson et al. 2000

The observations reveal a tight correlation between a dwarf’s star formation history, its mass, and its proximity to M31.

“We see that the duration for which the satellites can continue forming new stars really depends on how massive they are and on how close they are to the Andromeda galaxy,” said lead author Savino in a press release. “It is a clear indication of how small-galaxy growth is disturbed by the influence of a massive galaxy like Andromeda.”

Astronomers are in a difficult spot when it comes to studying galaxies in detail. Our own Milky Way is the only galaxy that’s open to detailed investigation. The temptation is to draw parallels between our knowledge of the MW and 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 principal investigator Daniel Weisz of the University of California at Berkeley. “There’s always been concerns about whether what we are learning in the Milky Way applies more broadly to other galaxies. Or is there more diversity among external galaxies? Do they have similar properties? 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.”

These detailed, 1,000-orbit observations of Andromeda are helping change this. They reveal a more chaotic environment than in the Milky Way.

“Everything scattered in the Andromeda system is very asymmetric and perturbed. It does appear that something significant happened not too long ago,” said Weisz.

One of the research’s surprising findings is that about half of M31’s dwarf galaxies lie along the same plane, called the Great Plane of Andromeda, and are moving in the same direction. “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,” said Weisz.

The galaxies along this plane don’t appear to be any different from those on the plane. “There is no difference between the median SFH (star formation history) of galaxies on and off the great plane of Andromeda satellites,” the authors write.

The researchers used colour-magnitude diagrams (CMDs), an important tool in astronomy, to learn more about the star formation history in Andromeda’s dwarf galaxies. CMDs plot a star’s magnitude, or brightness, with its colour. From these plots, astronomers can learn about the age of a stellar population and when star formation was quenched.

The CMDs showed that star formation in dwarf galaxies lasts much longer than expected. It started early and continued, albeit more slowly, by drawing from a reservoir of gas. These results are in sharp disagreement with simulations like TNG 50.

“Star formation really continued to much later times, which is not at all what you would expect for these dwarf galaxies,” said Savino. “This doesn’t appear in computer simulations. No one knows what to make of that so far.”

This figure from the team's research shows the star formation history (SFH) in Andromeda's halo, the Giant Stellar Stream, and M33. The red region represents the Epoch of Reionization, the black line shows the best-fit SFH and the grey shows systematic uncertainties. It shows that star formation started early and continued for a long time, albeit at a much slower rate. Image Credit: Savino et al. 2025. — This figure from the team’s research shows the star formation history (SFH) in Andromeda’s halo, the Giant Stellar Stream, and M33. The red region represents the Epoch of Reionization, the black line shows the best-fit SFH and the grey shows systematic uncertainties. It shows that star formation started early and continued for a long time, albeit at a much slower rate. Image Credit: Savino et al. 2025.

The research also shows that the SFH is no different between dwarf galaxies on the Great Plane of Andromeda and those off of it.

This figure from the study shows the median SFH for the GPoA candidate members (blue line, left panel) and out-of-plane candidates (orange line, middle panel). The gray lines show the SFH of individual galaxies. The right panel shows a direct comparison between the median SFH of the two samples. Image Credit: Savino et al. 2025.

The SFH results in Andromeda are not what we see in the MW. This suggests that the environmental histories, tidal forces, and gas stripping experienced by M31 satellites are different than those around the Milky Way, leading to different star formation patterns over cosmic time. This could be the most significant finding and further exemplifies the risk of extrapolating our knowledge of the Milky Way to other galaxies.

“The results of this study represent a significant step forward in our understanding of the M31 satellite system,” the authors write in their conclusion. They point out that the SFHs they’ve developed will only be more valuable when combined with large data sets acquired in the future. Data sets of the spectral abundance of stars and their proper motions in M31 are being acquired, and some already exist.

Maybe they’ll be able to explain Andromeda’s dwarf galaxies’ unusual properties.

“We do find that there is a lot of diversity that needs to be explained in the Andromeda satellite system,” added Weisz. “The way things come together matters a lot in understanding this galaxy’s history.”