Extended Stellar Components of Galaxies & the Nature of Dark Matter
  Deep observations of galaxies reveal faint extended stellar components
(hereafter ESCs) of streams, shells, and halos. These are a natural prediction
of hierarchical galaxy formation, as accreted satellite galaxies are tidally
disrupted by their host. We investigate whether or not global properties of the
ESC could be used to test of dark matter, reasoning that they should be
sensitive to the abundance of low-mass satellites, and therefore the underlying
dark matter model. Using cosmological simulations of galaxy formation in the
favoured Cold Dark Matter (CDM) and Warm Dark Matter (WDM) models ($m_{\rm
WDM}$=0.5,1,2 keV/$c^2$), which suppress the abundance of low-mass satellites,
we find that the kinematics and orbital structure of the ESC is consistent
across models. However, we find striking differences in its spatial structure,
as anticipated -- a factor of $\sim$10 drop in spherically averaged mass
density between $\sim$10% and $\sim$75% of the virial radius in the more
extreme WDM runs ($m_{\rm WDM}$=0.5, 1 keV/$c^2$) relative to the CDM run.
These differences are consistent with the mass assembly histories of the
different components, and are present across redshifts. However, even the least
discrepant of the WDM models is incompatible with current observational limits
on $m_{\rm WDM}$. Importantly, the differences we observe when varying the
underlying dark matter are comparable to the galaxy-to-galaxy variation we
expect within a fixed dark matter model. This suggests that it will be
challenging to place limits on dark matter using only the unresolved spatial
structure of the the ESC.
