The Andromeda Galaxy, a large neighbor of our Milky Way, looks like a faint, spindle-shaped object about the size of the full Moon when seen with the naked eye. It is 2.5 million light years away.
You can’t see from your backyard that Andromeda is surrounded by nearly three dozen small satellite galaxies, much like bees buzzing around a hive.
NASA’s Hubble Space Telescope studies this lively galactic “ecosystem” in amazing detail. Using more than 1,000 Hubble orbits, this ambitious survey took advantage of Hubble’s sharp, stable, and efficient optics. The project created a precise 3D map of the dwarf galaxies around Andromeda and traced their star formation over the 14 billion years of the universe’s history.
The Hubble Space Telescope revealed that the Andromeda Galaxy’s ecosystem differs from our Milky Way’s. This shows how these two galaxies have evolved over billions of years. While our Milky Way has had a calm history, Andromeda has had a much more dynamic past.
A major merger with another large galaxy a few billion years ago likely played a big role, and the fact that Andromeda is about twice as massive as our Milky Way could explain why it has so many diverse dwarf galaxies.
Most-detailed radio image of Andromeda galaxy to date
Surveying all of the Milky Way’s satellite galaxies is tricky because we’re inside it. Studying other large galaxies is also challenging because they’re too far away. Besides Andromeda, the closest galaxy comparable to the Milky Way is M81, nearly 12 million light-years away.
Having a “bird’s-eye view” of Andromeda’s satellite system helps us understand what drives the evolution of these tiny galaxies.
According to Alessandro Savino from the University of California at Berkeley, the ability of these satellite galaxies to form new stars depends on their mass and proximity to Andromeda. This shows how a massive galaxy like Andromeda influences the growth of smaller galaxies.
Daniel Weisz, also from the University of California at Berkeley, noted that the Andromeda system looks very asymmetric and disturbed, suggesting something significant happened not too long ago. He highlighted that we often apply what we learn from the Milky Way to other galaxies. Still, this work shows that low-mass galaxies in different environments can follow distinct evolutionary paths than those around the Milky Way.
Interestingly, about half of Andromeda’s satellite galaxies are confined to a plane and orbit in the same direction, which was a total surprise and remains something scientists don’t fully understand.
The brightest companion galaxy to Andromeda is Messier 32 (M32). This compact, ellipsoidal galaxy might be the remnant core of a larger galaxy that collided with Andromeda a few billion years ago. After losing gas and some stars, it continued its orbit. M32 has older stars, but there’s evidence of a star formation burst a few billion years ago.
In addition to M32, Andromeda has a unique population of dwarf galaxies you don’t see in the Milky Way. These dwarf galaxies formed most of their stars early on, but surprisingly, they kept forming stars at a low rate for much longer using a gas reservoir.
According to Alessandro Savino from the University of California at Berkeley, this continued star formation is unexpected for dwarf galaxies and doesn’t appear in computer simulations. No one knows what to make of this yet.
Daniel Weisz, also from the University of California at Berkeley, added that the Andromeda satellite system is very diverse and needs to be explained. Understanding how these galaxies come together is crucial to understanding Andromeda’s history.
Hubble provides the first set of images that let astronomers measure the motions of the dwarf galaxies. In about five years, Hubble or NASA’s James Webb Space Telescope will take a second set of observations. This will allow astronomers to do a dynamical reconstruction of all 36 dwarf galaxies, helping them to rewind and understand the motions of the entire Andromeda ecosystem billions of years into the past.
Journal Reference:
- Alessandro Savino, Daniel R. Weisz, Andrew E. Dolphin, Meredith J. Durbin, Nitya Kallivayalil, Andrew Wetzel, Jay Anderson, Gurtina Besla, Michael Boylan-Kolchin, Thomas M. Brown, James S. Bullock, Andrew A. Cole, Michelle L. M. Collins, M. C. Cooper, Alis J. Deason, Aaron L. Dotter, Mark Fardal, Annette M. N. Ferguson, Tobias K. Fritz, Marla C. Geha, Karoline M. Gilbert, Puragra Guhathakurta, Rodrigo Ibata, Michael J. Irwin, Myoungwon Jeon, Evan N. Kirby, Geraint F. Lewis, Dougal Mackey, Steven R. Majewski, Nicolas Martin, Alan McConnachie, Ekta Patel, R. Michael Rich, Evan D. Skillman, Joshua D. Simon, Sangmo Tony Sohn, Erik J. Tollerud, and Roeland P. van der Marel. The Hubble Space Telescope Survey of M31 Satellite Galaxies. IV. Survey Overview and Lifetime Star Formation Histories. The Astrophysical Journal. DOI 10.3847/1538-4357/ada24f
