The Andromeda Galaxy is the nearest large galaxy to the Milky Way, containing several times as many stars. Now, an international team of scientists has found out how it managed to reach such a huge size.
A few years ago, researchers led by an astrophysicist from the University of Sydney, Australia, discovered mysterious globular clusters on the outskirts of the Andromeda galaxy - the nearest large galaxy to us, located 800 kiloparsecs from the Milky Way - forming a stream they called the "Dalais structure" (Welsh for "black stream", the same name as the river and valley in Wales).
They have now discovered that these clusters are actually traces of the violent past of the Andromeda galaxy, which was increasing its size by swallowing smaller galaxies. Scientists have identified traces of at least two "feasts": one occurred relatively recently, during the last five billion years, while the other occurred much earlier, about eight to ten billion years ago, shortly after the formation of our universe.
Astronomers have become interested in the path of the Andromeda Galaxy because it is very similar to the Milky Way, but alas, it is not very convenient to observe our Galaxy from Earth. But we can observe the other galaxies unobstructed because they are not obscured by the stars and nebulae closest to us.
If the growth pattern of the Andromeda Galaxy is typical of large spiral galaxies, it is possible that the Milky Way, too, has in the past swallowed smaller galaxies before increasing dramatically in size. However, it is possible that our Galaxy may have had a different growth pattern which did not involve swallowing smaller galaxies; for instance it grew by "peacefully" sucking in interstellar material.
So far, the researchers have only been able to determine the speed and chemical composition of the globular clusters: they turned out to be noticeably poorer in metals than the surrounding stars. In the future, astronomers plan to estimate the distances between the individual structures and build a three-dimensional model of the flow, which will literally give a glimpse into the past of the Andromeda Galaxy, during its violent "feasts".