Last updated August 3, 2018 at 12:32 pm
The Milky Way had another sibling until it was eaten by the Andromeda galaxy.
Astronomers have revised the Milky Way’s family history after discovering the remains of a gigantic galaxy hiding in plain sight. The lost galaxy would have been the third largest in the local group which includes the Milky Way and Andromeda.
Wrapping around the Andromeda galaxy, the Milky Way’s larger sibling, is a vast collection of stars thought to be from a collision billions of years ago with a smaller cousin.
In a Nature Astronomy letter this collection of stars has been revealed to be all that’s left of a neighbouring galaxy decimated in an epic collision with the Andromeda galaxy around 2 billion years ago. Before the collision the galaxy would have been nearly half the size that our Milky Way is today.
A galaxy-eat-galaxy universe
Galaxies grow larger in two ways, one is the slow accretion of gas from intergalactic space that then slowly cools to create dense star-forming regions over time, the second is more direct – galactic cannibalism by direct accretion of neighbouring galaxies.
If the feeding galaxy happens to be merging with a neighbour at least a tenth its size then conventional wisdom would suggest it be structurally damaged in the act. These so-called major mergers results in the transformation of the delicate disk structure into a `train-wreck’ elliptical shape. The process would also lead to the gas in each galaxy being gravitationally agitated and rapidly converted in a gigantic starburst of star formation.
This latest research suggests that the conventional wisdom might not be quite right, and that merging galaxies don’t always obviously change as a result of such a titanic collision.
Tidal streams of stars
Deep observations of the region around Andromeda uncovered a vast collection of stars, a number of which form a stream wrapping around the galaxy. The stream was formed when the galaxy being consumed was torn apart by Andromeda’s gravity (much as the Moon is squeezed by Earth).
Furthermore, these stars wrapping around Andromeda are rich in heavy elements, in astronomy these are collectively termed metals, which tend to form in larger galaxies. Finally a tiny satellite galaxy of Andromeda, known as M32, is also vastly more metal-rich than would be expected for its size.
Using the Illustris supercomputer simulation researchers found that both the stream and the metal-rich nature of the stars in it could be explained by Andromeda merging with a galaxy 40% the stellar mass of the Milky Way.
The only issue is that the candidate M32 was at least ten times too small to have been the source. Instead, the simulations suggest M32 was a small part of a much larger galaxy that was disrupted by the gravity of Andromeda five billion years ago. The two galaxies finally merged 2 billion years ago, leaving behind only the stream of stars and its tiny central core, M32.
This larger galaxy was estimated to have 20 billion times the mass of our Sun in stars alone, putting it a close third to our Milky Way in the Local Group of neighbouring galaxies.
Such a major merger of large galaxies between the M32 progenitor and Andromeda would usually smash the delicate disk of the larger galaxy apart, while consuming the smaller galaxy entirely. In this case however, Andromeda did not seem to suffer any obvious major damage, and the smaller galaxy survived in a smaller form, making this a most untypical collision.
That Andromeda appears to have engaged in a recent bout of galactic cannibalism of a sibling of the Milky Way with barely any mess is a critical lesson for astronomers tracing the history of galaxies by measuring their structures today. It’s also a reminder that no siblings are safe in the Local Group.