Collapse of spherical overdensities in superfluid models of dark matter
  We intend to understand cosmological structure formation within the framework
of superfluid models of dark matter with finite temperatures. Of particular
interest is the evolution of small-scale structures where the pressure and
superfluid properties of the dark matter fluid are prominent. We compare the
growth of structures in these models with the standard cold dark matter
paradigm and non-superfluid dark matter. The equations for superfluid
hydrodynamics were computed numerically in an expanding $\Lambda$CDM background
with spherical symmetry; the effect of various superfluid fractions,
temperatures, interactions, and masses on the collapse of structures was taken
into consideration. We derived the linear perturbation of the superfluid
equations, giving further insights into the dynamics of the superfluid
collapse. We found that while a conventional dark matter fluid with
self-interactions and finite temperatures experiences a suppression in the
growth of structures on smaller scales, as expected due to the presence of
pressure terms, a superfluid can collapse much more efficiently than was
naively expected due to its ability to suppress the growth of entropy
perturbations and thus gradients in the thermal pressure. We also found that
the cores of the dark matter halos initially become more superfluid during the
collapse, but eventually reach a point where the superfluid fraction falls
sharply. The formation of superfluid dark matter halos surrounded by a normal
fluid dark matter background is therefore disfavored by the present work.
