1996A&A...315...52V

High resolution CO(1-0), CO(2-1) and optical observations of the highly inclined nearby galaxy NGC 7331 are presented. The analysis of the isophotal fit procedure to the optical images reveals a significant change of the position angle of the major axis as well as an oscillating behaviour of the a₄ Fourier coefficient in the inner 20", which is most probably due to the presence of a small-sized bar in the centre of the galaxy. Our CO observations do not prove directly the presence of a nuclear bar, however, the dynamics of the inner disk hint at the presence of a small, evolved bar depleted from molecular gas. The dynamical investigation of the very centre hints at a spiral structure within the inner, rigidly rotating disk. The molecular gas is concentrated on two extended maxima symmetrically located on either side of the dynamical centre of the galaxy, close to the turnover points of the rotation curve. A detailed investigation of the dynamical behaviour of this ring-like structure (diameter 80"≃5.4kpc) leads to the conclusion that these emission maxima originate rather from molecular clouds within spiral arms than from a homogeneous molecular ring. Furthermore, we have performed high-resolution numerical simulations of the spiral pattern evolution of NGC 7331. Using a mass distribution derived from the rotation curve of the galaxy, the results of the simulation are in very good accordance with both the dynamics and morphology of the CO and optical observations. The simulations reproduce the central gas hole as well as the locations of the ring-like enhancement of molecular material around the turnover points. We conclude that current models of dynamical galaxy evolution are able to reproduce the observed dynamical and morphological properties of this galaxy satisfactorily.