k-essence scalar dark matter solitons around supermassive black holes

We consider scalar dark-matter models where the theory has a shift symmetry only broken by the scalar mass term. We restrict ourselves to k-essence kinetic terms where the shift symmetric part of the Lagrangian is a function of the first derivatives of the scalar field only. When the scalar mass is much larger than the inverse of the astrophysical time and length scales of interest, these models provide a description of dark-matter equivalent to the one given by theories with only polynomial interactions, in the low-amplitude regime where the self-interactions are small contributions to the Lagrangian. In this regime and in the nonrelativistic limit, which apply on large galactic scales, scalar clouds form solitons with a finite core. This provides an adequate model for dark-matter halos with no singular behavior. Close to the center of galaxies, where a supermassive black hole (BH) resides, we analyze the scalar-field distribution and the fate of the dark-matter soliton when subject to the BH gravitational attraction. We show that the scalar-field profile around such a central BH can be described by new oscillatory solutions of a modified Klein-Gordon equation, which generalize the harmonic oscillations of free scalar dark matter in a flat environment and the Jacobi elliptic functions of the phi(4) model. Moreover, we find that, depending on the form of the k-essence kinetic term, regular solutions can be constructed or not, which connect the relativistic ingoing wavelike profile of the scalar field at the BH horizon to the nearly static nonrelativistic soliton at large distance. These profiles have a constant flux and represent the slow infall of scalar matter into the BH. We show that this regular behavior is only possible for k-essence functions that satisfy the usual conditions for the absence of ghosts and gradient instabilities, together with a new restriction on the growth of the kinetic function K(X) for large argument. It turns out that the same conditions of stability guarantee that quantum corrections are tamed, provided that the mass of the scalar field is less than 10(-3) eV and the strong coupling scale of the model Lambda is much larger than the scalar mass.