The orbital structure of galactic halos

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Abstract
Earlier numerical experiments have shown that the orbital structure in the halo of a triaxial logarithmic potential differs significantly from that in the main body of the potential; the usual box orbits are replaced by boxlets originating in higher resonances. Now we have further investigated this phenomenon, with the following results. First, as was shown earlier (e.g., Gerhard and Binney), a modified triaxial Hubble potential (rho is-proportional-to r-3) shows much the same replacement of boxes by boxlets in its halo as does the logarithmic potential (rho is-proportional-to r-2). This suggests that the dominant occurrence of boxlets may be a fairly general phenomenon in triaxial galactic halos. Second, numerical experiments in an oblate logarithmic potential show that most of the tube orbits in the halo are not affected by the higher resonances. Only the tube orbits which approach the center closely (high radial action and low latitudinal action) are replaced by tubelets. The latter orbits are not needed for self-consistent equilibrium solutions and therefore do not endanger the existence of such solutions for axisymmetric halos. Third, if the triaxiality of the density distribution is truncated at an appropriate distance (approximately 50 core radii), then at and outside the truncation the tube orbits are no longer elongated perpendicular to the figure; furthermore, the banana orbits, which are the most slender boxlets, are less thick and hence better match the model thickness. Both these consequences of triaxiality truncation favor the existence of self-consistent equilibria in galactic halos. Fourth, in halos with mild triaxiality (axis ratio c/a greater-than-or-similar-to 0.6), some of the boxlets are slender enough to match the model figure. Thus it seems likely that such halos have self-consistent equilibrium solutions. In contrast, in halos with strong triaxiality (c/a < 0.5) all boxlets are thicker than the model, making the existence of self-consistent equilibria for such halos questionable-at least as long as the density figure is not adjusted to the shape of the orbits. We accordingly estimate that, while the main bodies of galaxies in equilibrium may be strongly triaxial, the same may not be true for their halos. The triaxiality in the halo may have to be moderated or even truncated at the transition from main body to halo, i.e., at r almost-equal-to 10 kpc for a galaxy with a core radius of 200 pc. This estimate will have to be substantiated in the future by the numerical construction of equilibrium models which cover both the main body and the halo.

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