Galactic halos of fluid dark matter

Abstract

Dwarf spiral galaxies, and in particular the prototypical DDO 154, are known to be completely dominated by an unseen component. The putative neutralinos, so far the favored explanation for the astronomical dark matter, fail to reproduce the well measured rotation curves of those systems because these species tend to form a central cusp whose presence is not supported by observation. We have considered here a self-coupled charged scalar field as an alternative to neutralinos and investigated whether a Bose condensate of that field could account for the dark matter inside DDO 154 and more generally inside dwarf spirals. The size of the condensate turns out to be precisely determined by the scalar mass m and self-coupling $\ensuremath{\lambda}$ of the field. We find actually that for ${m}^{4}/\ensuremath{\lambda}\ensuremath{\sim}50--75{\mathrm{eV}}^{4}$ the agreement with the measurements of the circular speed of DDO 154 is impressive, whereas it lessens for larger systems. The cosmological behavior of the field is also found to be consistent, though marginally, with the limits set by big bang nucleosynthesis on the effective number of neutrino families. We conclude that classical configurations of a scalar and self-coupled field provide a possible solution to the astronomical dark matter problem and we suggest further directions of research.