An investigation of the structure and kinematics of the spiral galaxy NGC 6503

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Abstract
The spiral galaxy NGC 6503 exhibits a regular kinematical structure except for a remarkable drop of the stellar velocity dispersion values in the central region. To investigate the dynamics of the disc in general, and that of the central region in particular, a theoretical framework is described. This includes a mass decomposition of the galaxy into a family of disc/halo realizations compatible with the observed photometry and rotation curve. For this family, stellar velocity dispersion values and stability parameters have been calculated, showing that the more massive discs, although having larger dispersions, are less stable. However, a reliable theoretical description of the inner regions where the drop occurs cannot be given. We have therefore resorted to numerical calculations, not only to study the central region, but also to investigate the appearance of the disc in a general sense. Pure stellar three-dimensional simulations have been performed for the family of decompositions. A clear result is that disc/dark halo mass ratios approaching those of the maximum disc limit generate a large bar structure. This is incompatible with the observed morphology of NGC 6503. For radii larger than ∼ 0.2 scalelengths, the stellar kinematics resulting from the simulations essentially agrees with that predicted by the theory. Unfortunately, however, the central velocity dispersion drop cannot be reproduced. A close inspection reveals that the central nuclear region is very small and bright. Therefore, tentatively, this nucleus has been considered as an isothermal sphere and a core fitting procedure has been applied. For an adopted equal mass-to-light ratio of disc and nucleus, a velocity dispersion of 21.5 km s−1 is predicted, in excellent agreement with the observed central value. An analysis, in retrospect, of the local densities involved proves that the nucleus is local and gravitationally dominant such that its approximation as an isothermal sphere is justified. The observed dispersion drop can thus be explained by a separate kinematically distinct galactic component.
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