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

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3. Results

The model data cubes have been inspected and analyzed in the same way as the observed cubes. For the comparison of the models and the observations we have used the channel maps and the position-velocity maps along the major and minor axes. Here we present only the position-velocity maps along the major axis and only a small number of models, but we have explored the whole relevant parameter space.

Initially, we tried a model with a one-component gas layer. Cuts along the major axis for various thicknesses are shown in the first column of Fig. 2. The observations are in the top right panel. It is immediately clear that a thin disk model does not reproduce the observed asymmetry. A thickness of about 5 kpc (FWHM) would be needed to explain it. It is obvious that such a layer would be unrealistically thick, much thicker than that of our Galaxy, which according to Dickey and Lockman (1990) has a FWHM of only 220 pc, or that of NGC 891, for which most of the HI is in an unresolved layer with a FWHM thickness of less than 1 kpc (Swaters et al. 1997).

[FIGURE] Fig. 2. Position-velocity maps (receding side, see Fig. 1) along the major axis of NGC 2403 (PA=[FORMULA]) of the observations (top right panel, contour levels are -2.5, 2.5 (1.8[FORMULA]), 10, 20, 40, 60 and 100 mJy/beam) and of different models. The types of models are indicated at the top of each column. The FWHM values for the thin and thick components and their velocity differences are given in the panels. The angular and velocity resolutions are [FORMULA] and 8.2 km [FORMULA].

Therefore, we explored the possibility of having, instead of one very thick Gaussian disk, a relatively thin disk of high density and in addition a vertically more extended but lower density layer. For this we have built a model with two components, one thin and one thick, both rotating at the same velocity. The FWHM of the thick component and the column density ratio between the two components are free parameters. We found, however, that there is little freedom in the choice of this ratio and we fixed it tentatively at 1:1. With a lower density for the thick component only a weak asymmetry would appear. The best model seems the one with a thick component of FWHM close to 5 kpc (see Fig. 2). Hence this model is not significantly different from the one-component model and is equally unrealistic as it requires unreasonable amounts of HI at large distances from the plane.

Finally, we released the condition of corotation and explored the effects of a decrease of the rotational velocity with distance to the plane. For this we have constructed models with a slowly rotating thick component. Again, the FWHM of the thick component and the column density ratio are free parameters. But here as an additional parameter we have the velocity decrease. A thin-to-thick disk density ratio of 4:1 was found to give the best comparison with the data. With a higher ratio the asymmetry would tend to disappear, whereas a lower ratio would not reproduce the observed density contrast between the profile peak and its wings. For the thick component a FWHM of 1 kpc was adopted. For this thickness the best agreement with the observations is obtained with the 25 km [FORMULA] decrease (see Fig. 2). Models with a thick component of larger FWHM than 1 kpc were also tried. For example, in the lower right of Fig. 2, we show a model that also reproduces the observed asymmetry well, and in which the thick component, still with a 25 km [FORMULA] decrease in rotation velocity, has a thickness of 3.5 kpc. For this thickness the optimal column density ratio of the thin and thick disk is 6:1. This illustrates that it is difficult to distinguish between an extended, low density and a less extended, higher density vertical distribution.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000