Chris Power - May 10, 2017

What can the outskirts of galaxies tell us about dark matter?

Deep observations of galaxies reveal that they are embedded in faint extended
stellar structures, consisting of streams, shells, and halos. These extended
stellar structures are predicted by theoretical models of galaxy formation as a
natural consequence of hierarchical assembly -- gravitational tides acting on
lower mass systems lead to their disruption as they interact with their more
massive hosts during merging and accretion events. Because the merging timescale
for the lowest mass systems is long, they tend to undergo many interactions with
their host and so they are likely to spread tidal debris over the full extent of
their orbit. This is interesting because it suggests that the spatial extent of
extended stellar structures may differ in a systematic fashion between dark
matter models in which the abundance of low-mass systems are suppressed and the
canonical Cold Dark Matter (CDM) model, which is expected to have dark matter
small-scale structure to effectively arbitrarily low-masses. We report on our
exploration of this idea using a suite of cosmological zoom galaxy formation
simulations of an individual disc galaxy and study how the properties of the
extended stellar structure is likely to be influenced by the dark matter model,
using both CDM and Warm Dark Matter initial conditions. We comment on the
feasibility of testing this idea with future observational surveys.