7 The influence of the bar

The dynamical influence of the bar has been determined by calculating the potential field generated by the luminous distribution. To that aim the I-band image of NGC 3992 has been properly sky subtracted and defects and stars were interpolated over. The image was projected to face-on using a PA of 68$\degr$ and inclination of 57$\degr$ following from the H  I kinematics. As a bonus one can quite accurately measure the physical length of the bar in the face-on image which amounts to 145$\arcsec$. Next the potential image was calculated numerically and ellipses have been fitted to it. The resulting ellipticities $(\epsilon = 1-b/a)$as a function of the major axis of the ellipse are given in Fig. 11. When inspecting Fig. 11 it is obvious that the ellipticity of the bar potential is largest at a radius of 60$\arcsec$, at smaller radii the bulge makes the potential rounder while at larger radii the galactic disc takes over.

The bar ends at 72$\farcs$5 yet the extent of its elliptic potential does not reach beyond approximately 90$\arcsec$. As can be seen in Fig. 11, there is only a small overlap between the elliptic bar potential and presence of H  I gas, roughly between 70 and 90$\arcsec$, which is just at the position of the deviating patches of the velocity field bordering the central hole. It is therefore likely that these aberrations are indeed generated by the bar. On the other hand, judging the ellipticity of the potential, the bar will not generate a distortion of the velocity field for radii larger than 90$\arcsec$. Consequently the derived rotation curve is beyond dispute at those radii. Even at the positions further in, the residuals of the velocity field are so moderate that the rotational values are still reasonably well determined.

Figure 11: The crosses give the ellipticity of the potential as generated by the I-band image which was projected to face-on. The influence of the bar barely reaches into the regions where gas is present (dashed line).