Structure and kinematics of edge-on galaxy discs -- V. The dynamics of the stellar discs

Abstract

In earlier papers in this series we determined the intrinsic stellar disc kinematics of fifteen intermediate to late type edge-on spiral galaxies using a dynamical modeling technique. From the photometry we find that intrinsically more flattened discs tend to have a lower face-on central surface brightness and a larger dynamica mass-to-light ratio. This observation suggests that at a constant maximum rotational velocity lower surface brightness discs have smaller vertical stellar velocity dispersions.Although the individual uncertainties are large, we find from the dynamical modeling that at least twelve discs are submaximal. The average disc contributes 53$\pm$4 percent to the observed rotation at 2.2 disc scalelengths, with a 1$σ$ scatter of 15 percent. This percentage becomes somewhat lower when effects of finite disc flattening and gravity by the dark halo and the gas are taken into account. Since boxy and peanut-shaped bulges are probably associated with bars, the result suggests that at 2.2$h_{\rm R}$ the submaximal nature of discs is independent of barredness. The possibility remains that very high surface brightness discs are maximal.We confirm that the radial stellar disc velocity dispersion is related to the galaxy maximum rotational velocity. The scatter in this $σ-v_{\rm max}$ relation appears to correlate with the disc flattening, face-on central surface brightness and dynamical mass-to-light ratio. Low surface brightness discs tend to be more flattened and have smaller stellar velocity dispersions. The findings are consistent with the observed correlation between disc flattening and dynamical mass-to-light ratio.