Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 451, Issue 1, Pages 601-610Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stv844
Keywords
galaxies: elliptical and lenticular, cD; galaxies: kinematics and dynamics; galaxies: nuclei
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Funding
- German Research Foundation (DFG) [Sonderforschungsbereich SFB 881]
- RFBR [14-05-00080, 15-52-12387]
- Department of Physical Sciences of the Royal Astronomical Society (RAS) [OFN-17]
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We consider a two-parametric family of radially anisotropic models with non-singular density distribution in the centre. If highly eccentric orbits are locked near the centre, the characteristic growth rate of the instability is much less than the Jeans and dynamic frequencies of the stars (slow modes). The instability occurs only for even spherical harmonics and the perturbations are purely growing (aperiodic). On the contrary, if all orbits nearly reach the outer radius of the sphere, both even and odd harmonics are unstable. Unstable odd modes oscillate having characteristic frequencies of the order of the dynamical frequencies (fast modes). Unstable even harmonics contain a single aperiodic mode and several oscillatory modes, the aperiodic mode being the most unstable. The question of the nature of the radial orbit instability (ROI) is revisited. Two main interpretations of ROI have been suggested in the literature. The first refers to the classical Jeans instability associated with the lack of velocity dispersion of stars in the transverse direction. The second interpretation refers to Lynden-Bell's orbital approach to bar formation in disc galaxies, which implies slowness and bi-symmetry of the perturbation. Oscillatory modes, odd spherical harmonics modes and non-slow modes found in one of the models show that the orbital interpretation is not the only possible interpretation.
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