Neutralino Dark Matter vs Galaxy Formation

Neutralino dark matter may be incompatible with current cold dark matter models with cuspy dark halos, because excessive synchrotron radiation may originate from neutralino annihilations close to the black hole at the galactic center. We report results obtained in [1], to which we refer for further details. The composition of dark matter is one of the major issues in cosmology. A popular candidate for non-baryonic cold dark matter is the lightest neutralino appearing in a large class of supersymmetric models [2]. In a wide range of supersymmetric parameter space, relic neutralinos from the Big Bang are in principle abundant enough to account for the dark matter in our galactic halo [3]. A generic prediction of cold dark matter models is that dark matter halos should have steep central cusps, meaning that their density rises as r to the center. Semi-analytical calculations find a cusp slope γ between ∼ 1 [4] and 2 [5]. Simulations find a slope γ ranging from 0.3 [6] to 1 [7] to 1.5 [8]. It is unclear if dark matter profiles in real galaxies and galaxy clusters have a central cusp or a constant density core. There is mounting evidence that the non-thermal radio source Sgr A at the galactic center is a black hole of mass M ∼ 3 × 10 M⊙. This inference is based on the large proper motion of nearby stars [9], the spectrum of Sgr A (e.g. [10,11]), and its low proper motion [12]. It is difficult to explain these data without a black hole [13]. The black hole at the galactic center modifies the distribution of dark matter in its surroundings [14], creating a high density dark matter region called the spike – to distinguish it from the above mentioned cusp (see Fig. 1 for an illustration). Signals from particle dark matter annihilation in the spike may be used to discriminate between a central cusp and a central core. With a central cusp, the annihilation signals from the galactic center increase by many orders of magnitude. With a central core, the annihilation signals do not increase significantly. Stellar winds are observed to pervade the inner parsec of the galaxy [10], and are supposed to feed the central black hole (e.g. [11,15]). These winds carry a magnetic field whose measured intensity is a few milligauss at a distance of ∼ 5pc from the galactic center [16]. The magnetic field intensity can rise to a few kilogauss at the Schwarzschild radius of the black hole in some accretion models for Sgr A [17]. The existence and strength of a magnetic field in the inner parsec of the galaxy is crucial to our argument.