Dynamical friction in Bose-Einstein condensed self-interacting dark
  matter at finite temperatures, and the Fornax dwarf spheroidal
  The aim of the present work is to better understand the gravitational drag
forces, i.e. dynamical friction, acting on massive objects moving through a
self-interacting Bose-Einstein condensate, also known as a superfluid, at
finite temperatures. This is relevant for light scalar models of dark matter
with weak self-interactions that require nonzero temperatures, or that have
been heated inside galaxies. We derived expressions for dynamical friction
using linear perturbation theory, and compared these to numerical simulations
in which nonlinear effects are included. After testing the linear result, it
was applied to the Fornax dwarf spheroidal galaxy, and two of its
gravitationally bound globular clusters. Dwarf spheroidals are well-suited for
indirectly probing properties of dark matter, and so by estimating the rate at
which these globular clusters are expected to sink into their host halo due to
dynamical friction, we inferred limits on the superfluid dark matter parameter
space. The dynamical friction in a finite-temperature superfluid is found to
behave very similarly to the zero-temperature limit, even when the thermal
contributions are large. However, when a critical velocity for the superfluid
flow is included, the friction force can transition from the zero-temperature
value to the value in a conventional fluid. Increasing the mass of the
perturbing object induces a similar transition to when lowering the critical
velocity. When applied to two of Fornax's globular clusters, we find that the
parameter space preferred in the literature for a zero-temperature superfluid
yields decay times that are in agreement with observations. However, the
present work suggests that increasing the temperature, which is expected to
change the preferred parameter space, may lead to very small decay times, and
therefore pose a problem for finite-temperature superfluid models of dark
matter.
