Density cusps: restrictions on non-axisymmetric models |
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Authors: | Syer & HongSheng Zhao |
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Institution: | Max-Planck-Institut für Astrophysik, 85740 Garching-bei-München, Germany,;Sterrewacht Leiden, Niels Bohr Weg 2, Leiden, the Netherlands |
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Abstract: | Galactic nuclei are now generally thought to have density cusps in their centres, and the evidence is mounting that as a consequence they are unlikely to be triaxial. Self-consistent stellar dynamical models of non-axisymmetric cusps would be an interesting counter-argument to this conclusion. We consider 2D analogues of triaxial cusps: a sequence of non-axisymmetric, cuspy discs first described by Sridhar & Touma. Scale-free models with potential Φ ∝ r α are examined in detail. It is shown analytically for 0 < α ? 0.43 that self-consistent models with positive phase-space density do not exist. Numerical solutions of the combined Vlasov and Poisson equations suggest that the whole sequence of models with 0 < α < 1 are also unphysical. Together with existing work on cusps, we conclude on purely theoretical grounds that galactic nuclei are not expected to be triaxial. |
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Keywords: | galaxies: kinematics and dynamics |
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