Could pressureless dark matter have pressure?
  A two-fluid dark matter model, in which dark matter is represented as a
two-component fluid thermodynamic system, without interaction between the
constituent particles of different species, and with each distinct component
having a different four-velocity, was recently proposed in Harko & Lobo, [Phys.
Rev. D83, 124051 (2011)]. In the present paper we further investigate the
two-fluid dark matter model, by assuming that the two dark matter components
are pressureless, non-comoving fluids. For this particular choice of the
equations of state the dark matter distribution can be described as a single
anisotropic fluid, with vanishing tangential pressure, and non-zero radial
pressure. We investigate the properties of this model in the region of constant
velocity galactic rotation curves, where the dynamics of the test particles is
essentially determined by the dark matter only. By solving the general
relativistic equations of mass continuity and hydrostatic equilibrium we obtain
the geometric and physical parameters of the dark matter halos in the constant
velocity region in an exact analytical form. The general, radial coordinate
dependent, functional relationship between the energy density and the radial
pressure is also determined, and it differs from a simple barotropic equation
of state.
