The shape of the velocity ellipsoid in NGC 488

Theories of stellar orbit diffusion in disc galaxies predict different rates of increase of the velocity dispersions parallel and perpendicular to the disc plane, and it is therefore of interest to measure the different velocity dispersion components in galactic discs of different types. We show that it is possible to extract the three components of the velocity ellipsoid in an intermediate-inclination disc galaxy from measured line-of-sight velocity dispersions on the major and minor axes. On applying the method to observations of the Sb galaxy NGC 488, we find evidence for a higher ratio of vertical-to-radial dispersion in NGC 488 than in the solar neighbourhood of the Milky Way (the only other place where this quantity has ever been measured). The difference is qualitatively consistent with the notion that spiral structure has been relatively less important in the dynamical evolution of the disc of NGC 488 than molecular clouds.