Reconstructing the accretion history of Andromeda

Large galaxies are surrounded by diffuse envelopes of stars that extend to very large radial distances. These stellar halos are a natural consequence of hierarchical formation processes, where the growth of large galaxies is powered by the continual accretion and destruction of smaller systems. Locating and measuring the stellar streams from such events in principle allows the accretion history of a galaxy to be reconstructed. Moreover, the orbits of such streams can be used to accurately infer the properties of the underlying gravitational field, which is dominated by the influence of the host galaxy’s dark matter halo.

Andromeda (M31) is the nearest large spiral galaxy to our own, and is often considered to be the “twin sister” of the Milky Way. The Pan-Andromeda Archaeological Survey (PAndAS) — a major international collaboration of astronomers — has detected a plethora of stellar streams in its halo. These sub-structures point to a vigorous history of dwarf satellite disruption, in qualitative agreement with predictions from cosmological models. We are looking for ambitious students to join the project. You will use data from the Hubble Space Telescope and 10m Keck Telescope together with numerical modelling to investigate the following questions: (i) how many satellites have been accreted and destroyed by Andromeda; (ii) when did these accretion events occur; (iii) what were the properties of the now-defunct dwarfs; and (iv) where did they come from? An additional key aim will be to measure the shape, size, and extent of Andromeda’s dark matter halo.