  The rotation curves of spiral galaxies are one of the basic predictions of
the cold dark matter paradigm, and their shape in the innermost regions has
been hotly debated over the last decades. The present work shows that dark
matter annihilation into electron-positron pairs may affect the observed
rotation curve by a significant amount. We adopt a model-independent approach,
where all the electrons and positrons are injected with the same initial energy
E_0 ~ m_dm*c^2 in the range from 1 MeV to 1 TeV and the injection rate is
constrained by INTEGRAL, Fermi, and HESS data. The pressure of the relativistic
electron-positron gas is determined by solving the diffusion-loss equation,
considering inverse Compton scattering, synchrotron radiation, Coulomb
collisions, bremsstrahlung, and ionization. For values of the gas density and
magnetic field that are representative of the Milky Way, it is estimated that
pressure gradients are strong enough to balance gravity in the central parts if
E_0 < 1 GeV. The exact value depends somewhat on the astrophysical parameters,
and it changes dramatically with the slope of the dark matter density profile.
For very steep slopes, as those expected from adiabatic contraction, the
rotation curves of spiral galaxies would be affected on ~kpc scales for most
values of E_0. By comparing the predicted rotation curves with observations of
dwarf and low surface brightness galaxies, we show that the pressure from dark
matter annihilation may improve the agreement between theory and observations
in some cases, but it also imposes severe constraints on the model parameters
(most notably, the inner slope of halo density profile, as well as the mass and
the annihilation cross-section of dark matter particles into electron-positron
pairs).
