Rotation curves and orbits in the scalar field dark matter halo
  spacetime
  The spacetime satisfying the flat curve condition for galaxies is shown to be
the zero mass limit of the dilaton blackhole with an exponential potential. We
derive the geodesic equations and by studying rotational curves and light
deflection, and find that the presence of the dilaton increases the
gravitational effects overall but diminishes the contribution from the central
mass when the distances are close to the black hole. Also, the dilaton reduces
the size of innermost stable circular time-like orbits, while the radius of the
photon sphere may be larger or smaller than its Schwrazschild counterpart,
depending on the strength of the dilaton field. We also show that a
generalisation of the flat-curve-condition spacetime considered elsewhere in
the literature does not solve the Einstein equation.
