The Archaeological Record in Local Galaxy Stellar Halos

Arguably one of the most fundamental problems in astrophysics is understanding the processes by which galaxies form and evolve. While sophisticated computer simulations can give an outline of the processes involved, they do not provide the full picture. For this, we must rely on detailed observational studies of galaxies across cosmic time.

Observations of distant galaxies show us snapshots of how these systems looked in their youth. This angle has recently gained new impetus due to JWST's exciting discovery of "baby galaxies" at surprisingly early times after the Big Bang. Nevertheless, it remains challenging to understand the evolutionary pathways of these systems after the time at which they are observed. Inferences necessarily rely on models and there are often many uncertainties.

An alternate approach to studying galaxy formation and evolution is to look in detail at the very nearest systems, including the Milky Way galaxy in which we live. The old and intermediate-age stars in these systems formed many billions of years ago and through analysing their spatial patterns, heavy element contents and motions, we have a powerful means to reconstruct their entire histories across cosmic time. This "archaeological record" has been very difficult to extract in the past due to the challenge of obtaining the required observational datasets, but major breakthroughs thanks to new facilities are now making the research possible.

Our current understanding of the galaxy formation process suggests that it is driven by hierarchical growth - in other words, large galaxies are built up over time by cannibalising many smaller systems. The most obvious signatures of this violent past is predicted to lurk in the form of vast stellar halos - populated largely by "immigrant'' stars and globular star clusters - and long-lived star streams from recently-disrupted dwarf galaxies. In this project, we will search for and study the fossil record of galaxy formation that is buried in the optically dim outskirts of a sample of a few tens of the nearest galaxies.

The research will provide unparalleled insight into the detailed histories of individual galaxies and will allow us to address a particularly outstanding question: has our own Milky Way formed in a normal or abnormal way? Indeed, recent observations from the Gaia satellite indicate that the Milky Way had its last significant "galactic meal" about 10 billion years ago, while our neighbouring galaxy, Andromeda, has continued to feast until much more recently. This project will finally settle the question of which one of these feeding habits is more typical of disc galaxies in general.