A light scalar field at the origin of galaxy rotation curves

The nature of the dark matter that binds galaxies remains an open question. It is usually assumed to consist in a gas of massive particles with evanescent interactions; however, such particles-which have never been observed directly-should have a clumpy distribution on scales less than or equal to 10(-2) kpc, which may be in contradiction with observations. We focus here on an exotic dark matter candidate: a light non-interacting (or only self-interacting) complex scalar field. We investigate the distribution of the field in gravitational interaction with matter, assuming no singularities (like black holes) at the galaxy center. This simplistic model accounts quite well for the rotation curve of low-luminosity spirals. A chi-squared analysis points towards a preferred mass m similar to 0.4 to 1.6 X 10(-23) eV in absence of self-interaction. A rough calculation shows that allowing for a quartic self-coupling may shift the upper bound to around 1 eV. We conclude that a scalar field is a promising candidate for galactic dark matter. Our comparison should be extended to other rotation curves in order to derive better constraints on the scalar potential. We finally give a hint of the issues that appear when one tries to implement this scenario on cosmological time scales. (C) 2002 Elsevier Science B.V. All rights reserved.