The kinematics of the bulge and the disc of NGC 7331

Results are presented of spectroscopic emission and absorption line observations along the major axis of the Sb galaxy NGC 7331. The kinematics of the ionized gas and stellar component are derived, being regular and symmetric with respect to the centre of the galaxy. Contrary to what may be expected, for R <∼ 40′′ the emission line gas appears to rotate slower than the stars. A likely explanation for this phenomenon is an inclined and warped gas layer in those inner regions. In the bulge dominated region the absorption line profiles are asymmetric in the sense that they have a shallow extension towards the systemic velocity. No counterrotating stellar component is observed, which is contrary to previous claims. As demonstrated, these claims might be based on a wrong interpretation of the employed analysis method. Outside the bulge dominated region the stellar radial velocities are in agreement with the neutral hydrogen dynamics and the stellar velocity dispersion decreases towards larger radii. A detailed bulge/disc light decomposition has been made. This has been used to construct a kinematical model of NGC 7331 from which model absorption line profiles were calculated. These profiles have been compared with the observations and model parameters have been adjusted to obtain a good match. 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 the stellar velocity dispersions and photometry result in a mass-to-light ratio of 1.6 ± 0.7 M /L . This value agrees with previous determinations for other discs using observed velocity dispersions. A rotation curve analysis allows the calculation of the mass-to-light ratio of the bulge which amounts to 6.8 in the I-band; considerably larger than the disc value. It appears that the mass distribution of NGC 7331 is completely dominated by the combination of bulge and dark halo at all radii. Comparing well determined mass-to-light ratios of a number of bulges with disc values (M/L)bulge is, on average, three times as large as (M/L)disc in the I-band. For the B-band this ratio goes up to 7.2, a fact which should have cosmological consequences.