How did large galaxies, like our own Milky Way and the nearby Andromeda Galaxy, emerge from the featureless soup that existed after the birth of our universe?
Decades of observations and theoretical effort have provided a picture in which atoms pool into galaxies, drawn by the gravitational pull of dark matter. Our synthetic universes, created and evolved on the most powerful supercomputers, beautifully match the distribution of galaxies we see on the universe.
Understanding the small-scale detail is going to occupy astronomers for decades to come, but it appears that galaxies are built up over time by accreting (gathering) smaller systems that stray too close.
And in the past few decades, it has become clear that the tenuous stellar halo that surrounds large galaxies holds the clues to unravelling galaxies accretion history.
A big spiral galaxy can be split into three key pieces: the bulge, the disk and the halo.
The bulge and disk are home to the vast majority of the hundreds of billions of stars that make up the galaxy, whereas only 1% of the stars can be found in the halo that envelopes a galaxy.
The Conversation, CC BY-ND
The halo hosts some of the oldest stars to be found in a galaxy. It is also home to globular clusters, the oldest bound groups of stars we know in the universe.
These suggest that the halo was the first galactic component to form, and it should hold some of the best preserved record for the formation history of a galaxy.
But to reveal the secrets hidden within a galaxy, astronomers have to take a forensic approach and look at the stellar halo like a crime scene.