Andromeda may have consumed the Milky Way’s long-lost sibling

Key takeaways:

1. About 2 billion years ago, the Andromeda Galaxy absorbed a massive galaxy called M32p, which transformed into the compact, unusual satellite galaxy M32 that we see orbiting Andromeda today.
2. This collision made Andromeda’s stellar halo dense with metal-rich, intermediate-age stars, and triggered a major burst of star formation, shaping Andromeda’s thickened disk.
3. The merger theory suggests Andromeda’s current mass and structure resulted from a single large collision rather than multiple small mergers over time.
4. M32 is unique among galaxies, as it’s both incredibly compact and contains many young stars, unlike typical old, elliptical galaxies.
5. The study challenges previous ideas about galactic collisions, suggesting that spiral galaxy disks, like Andromeda’s, are more resilient to major impacts than previously thought.

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A  study published in Nature Astronomy reveals that the Andromeda Galaxy, the largest galaxy in the Local Group, may have “cannibalized” a neighboring galaxy about 2 billion years ago, transforming it into its current compact satellite galaxy, M32. This study suggests that Andromeda once collided with a large companion galaxy known as M32p, stripping it of much of its mass and leaving only a fraction of its original size. This discovery reveals new aspects of Andromeda’s development and offers insights into galaxy formation on a grand scale.

A Colossal Collision with Andromeda’s Neighbor

With around a trillion stars’ worth of mass, Andromeda exerts a powerful gravitational pull that has drawn in and disrupted numerous smaller galaxies over its history. Through cosmological simulations, the study suggests that Andromeda’s mass and unique stellar halo structure are best explained by a single major merger with M32p, which was once the third-largest galaxy in the Local Group. This former giant, estimated to be 20 times the mass of any galaxy merged with the Milky Way, ultimately became the unusual and highly compact galaxy known as M32.

M32 stands out among galaxies as an oddity, appearing as a compact elliptical galaxy with an unusually high concentration of young stars. “M32 is a weirdo,” said study co-author Eric Bell of the University of Michigan, highlighting how the galaxy’s dense, youthful star population differentiates it from other elliptical galaxies. According to Bell, understanding M32’s origins could unravel key mysteries about galaxy evolution.

The massive collision with M32p likely provided the Andromeda Galaxy with its rich halo of metal-laden, intermediate-aged stars, as well as its thickened disk structure. This event may have triggered a burst of star formation about 2 billion years ago when approximately 20% of Andromeda’s stars were born. Bell noted that Andromeda would have looked dramatically different during this active period, filled with newly forming stars.

Implications for Galactic Evolution

These findings not only offer an in-depth look into Andromeda’s formation but also raise new questions about galaxy resilience in the aftermath of a major collision. The preservation of Andromeda’s spiral disk structure after such a large impact suggests that galactic disks may be more durable than previously believed. This resilience challenges previous theories, which assumed that galactic disks would be disrupted following such major mergers.

Looking forward, scientists hope to apply similar simulation techniques to study other galaxies and further understand how mergers have shaped their growth over time. This knowledge is especially relevant as Andromeda itself approaches a collision with the Milky Way in about 4 billion years. Studying past galactic interactions, like that of Andromeda and M32p, could provide astronomers with crucial insights into what might happen when these two galactic giants eventually merge.