We present a scenario for galaxy formation based on the hypothesis of scalar field dark matter. We interpret galaxy formation through the collapse of a scalar field fluctuation. We find that a cosh potential for the self-interaction of the scalar field provides a reasonable scenario for galactic formation, which is in agreement with cosmological observations and phenomenological studies in galaxies. 04.25.Dm, 95.30.Sf, 95.35.+d, 98.62.Ai, In the last years, the quest concerning the nature of the dark matter in the Universe has received much attention and has become of great importance for understanding the structure formation in the Universe. Some candidates for dark matter have been discarded and some others have recently appeared. The standard candidates of the Cold Dark Matter (CDM) model are axions and WIMP'S (Weakly Interacting Massive Particles), which are themselves not free of problems. Axions are massive scalar particles with no self interaction. In order for axions to be an essential component of the dark matter content of the Universe, their mass should be m ∼ 10 −5 eV . With this axion mass, the scalar field collapses forming compact objects with masses of order of M crit ∼ 0.6m 2 P l /m ∼ 10 −6 M ⊙ [1,2], which corresponds to objects with the mass of a planet. Since the dark matter mass in galaxies is ten times higher than the luminous matter, we would need tenths of millions of such objects around the solar system, which is clearly not the case. On the other hand, there are many viable particles with nice features in super-symmetric theories, such as WIMP'S. However, since these candidates behave just like standard CDM, they can not explain the observed scarcity of dwarf galaxies and the smoothness of the galactic-core matter densities, since high resolution numerical simulations with standard CDM predict an excess of dwarf galaxies and density profiles with cusps [3]. This is the reason why we need to look for alternative candidates that can explain both the structure formation at cosmological level, the observed amount of dwarf galaxies, and the dark matter density profile in the core of galaxies.In a recent series of papers, we have proposed that the dark matter in the Universe is of a scalar field nature with a strong self-interaction [4][5][6][7][8]. The scalar field has been proposed as a viable candidate, since it mimics standard CDM above galactic scales very well, reproducing most of the features of the standard Lambda Cold Dark Matter (ΛCDM) model [6,7,9,10]. However, at galactic scales, the scalar field model presents some advantages over the standard ΛCDM model. For example, it can explain the observed scarcity of dwarf galaxies since it produces a sharp cut-off in the Mass Power Spectrum. Also, its selfinteraction can, in principle, explain the smoothness of the energy density profile in the core of galaxies [7,11]. Nevertheless, the main problem when a new dark matter candidate is proposed is the study of the final object that would be formed as a result of a gravitational collapse.The...

Abstract. We present a general description of the scalar field dark matter (SFDM) hypothesis in the cosmological context. The scenario of structure formation under such a hypothesis is based on Jeans instabilities of fluctuations of the scalar field. It is shown that it is possible to form stable long lived objects consisting of a wide range of typical galactic masses around 10 12 M once the parameters of the effective theory are fixed with the cosmological constraints. The energy density at the origin of such an object is smooth as it should.

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We use the harmonic maps ansatz to find exact solutions of the Einstein-Maxwell-Dilaton-Axion (EMDA) equations. The solutions are harmonic maps invariant to the symplectic real group in four dimensions Sp(4, ) ∼ O(5). We find solutions of the EMDA field equations for the one and two dimensional subspaces of the symplectic group. Specially, for illustration of the method, we find space-times that generalise the Schwarzschild solution with dilaton, axion and electromagnetic fields.