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Astron. Astrophys. 356, L49-L52 (2000)

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2. Modelling

We consider the possibilities of both a thick HI disk (as opposed to the usually assumed thin layer), and of a two-component structure, with a thin layer and a thicker but less dense one. This thick layer may be corotating with the thin one or may be rotating more slowly. Both cases are examined. The reason for considering the slower rotation is that a non-corotating disk and halo have been proposed by Swaters et al. (1997) as the most likely interpretation for the HI observations of the edge-on galaxy NGC 891.

Axial symmetry and circular motions are assumed throughout. This assumption is supported by the results of Schoenmakers et al. (1997) who found from their harmonic analysis of the HI velocity field of NGC 2403 that non-circular motions are not important in this galaxy. Therefore, we have modelled NGC 2403 with a set of concentric rings, each ring characterized by a radius, a circular velocity, a velocity dispersion and a column density. Centre, systemic velocity, inclination and position angle, scaleheight and vertical density profile were chosen to be the same for all rings. The values adopted here are, unless specified otherwise, those derived by Sicking (1997).

The thin component always has a FWHM thickness of 0.5 kpc, the thickness of the thick layer is a free parameter. For the vertical density profile we have assumed a Gaussian distribution. We have also tried an exponential distribution, but the results do not seem to change significantly. In the case of non-corotation the circular velocity of the thicker component has been lowered by a fixed amount, for example 25 km [FORMULA], in the flat part of the curve and a proportionally smaller one in the inner parts. For the velocity dispersion we have used a value of 7 km [FORMULA] as derived for the profile widths in the regions of the flat part of the rotation curve. We have also constructed models with higher velocity dispersions for the thick components, up to 30 km [FORMULA], but the results do not differ significantly.

The two-component models have been constructed by adding two one-component models. This is obviously an oversimplification as it is more likely that any vertical decrease of density and velocity would be gradual.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000