It wasn’t just the Milky Way and Andromeda two billion years ago.
Here in the Local Group, our two largest galaxies dominate: Andromeda and the Milky Way.
The next largest, Triangulum, is much smaller: only 5% the Milky Way’s mass.
About 60 dwarf galaxies also remain: the cosmic survivors.
But the history of galactic cannibalism, where larger galaxies devour smaller ones, endures.
Two of the Milky Way’s larger satellites — the Magellanic Clouds — are interacting, forming stars, and on track to be devoured.
But one of Andromeda’s satellites is even more interesting.
M32 is the smallest galaxy in the Messier catalog: just 6,500 light years across, with ~3 billion solar masses of material.
Its dense core houses a multi-million solar mass black hole, extremely unusual for a small galaxy.
It suggests that M32 was once much larger, and has been partially cannibalized.
Scientists Richard D’Souza and Eric Bell pieced together:
- a large halo of stars,
- a stellar stream,
- and M32, replete with young stars,
to reveal a progenitor galaxy: M32p.
It was devoured by Andromeda 2 billion years ago; only the core survives.
This explains the large population of ~2 billion year old stars in both galaxies.
This reconstruction portends the ultimate, post-merger fate of the local group.
Mostly Mute Monday tells the astronomical story of an object, phenomenon, or scientific find about our Universe in images, visuals, and no more than 200 words. Talk less; smile more.Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.