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Astron. Astrophys. 317, 36-42 (1997)

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4. Summary

Fitting tilted ring models to the H I velocity data for NGC 1300 in the radial interval [FORMULA] we find [FORMULA] and [FORMULA]. Since error analysis is overly complex and involves several cross-correlated parameters, our quoted errors are based on visual estimates. Using a wedge when determining the orientation parameters gives similar results, but with larger errors.

For systems having a low inclination angle, there is an inherent difficulty when estimating the inclination angle. Due to projection, large variations of a low inclination angle only cause small changes in the sampling of the velocity data, e.g. when fitting tilted rings. This implies that for decreasing inclinations it becomes more difficult to disentangle the rotational velocity V from the inclination angle i. Thereby the analysis becomes increasingly insensitive to the inclination angle as the inclination angle decreases, a well known effect previously discussed by e.g. Begeman (1987, 1989). Begeman found that it was difficult to determine rotation curves for inclinations less than [FORMULA], while we in this particular case find the rotation curve to be derivable at an inclination of [FORMULA].

When analyzing the rotation curve, one has to be aware of the limitations. We can only trust the rotation curve when we are sufficiently far from the bar. Furthermore, limited resolution causes "beam- smearing" in regions with strong velocity gradients, e.g. the nuclear region. We find the inner [FORMULA] of the observed H I velocity field to be severely affected by beam-smearing, and a better representation in the nuclear region is given by the combined H I and optical velocity field in Fig. 9. Therefore, the inner part of the rotation curve in Fig. 8 is better described by the optical measurements.

Dust lanes are commonly believed to be regions of strong gas compression and strong velocity gradients (e.g. Athanassoula 1992; Lindblad P.A.B. et al. 1996). The new optical spectrum RED2 presented here reveals a projected velocity gradient across the dust lanes of 20-30 km s[FORMULA], not detected in the H I data, showing the relevance of high-resolution velocity data in the bar region.

There is strong evidence for global non-circular motions in the bar region of NGC 1300 as shown by the H I velocity residual map. Furthermore, there is a strong correlation between the residual velocities and the spiral features, with the largest residuals (-20 km s[FORMULA] ) in the spiral arm situated just south of the bar.

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© European Southern Observatory (ESO) 1997