The kinematics of the bulge and the disc of NGC 7331

Abstract

Presented are spectroscopic emission and absorption line observations along the major axis of the Sb galaxy NGC 7331. The kinematics of the ionized gas and the stars appears to be regular, but contrary to what one might expect, the emission line gas rotates slower than the stars in the inner regions. This may be caused by an inner inclined and warped gas layer. In the bulge region the absorption line profiles have a shallow extension towards the systemic velocity, but no counterrotation is observed which is contrary to previous claims. These claims might have been based on a wrong interpretation of the employed analysis method. A kinematical model has been made in order to explain the observed sizes and shapes of the absorption line profiles. It appeared necessary to combine a rapidly rotating disc having a radially decreasing velocity dispersion with a slowly rotating constant dispersion bulge. Then, simultaneously, the observed stellar radial velocities, the velocity dispersions and the observed asymmetry of the line profile could be explained satisfactorily. An even better fit to the data can be achieved when the disc is relatively thinner and colder inside the bulge region. For the disc a M/L ratio of 1.6 +/- 0.7 is derived in the I band. On the other hand, a rotation curve fit gives a M/L ratio of 6.8 +/- 1 for the bulge. An analysis of a sample of galactic discs and bulges shows that on average the mass-to-light ratio of the bulge is three times as large as the mass-to-light ratio of the disc in the I band. For the B-band this ratio goes up to 7.2.