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

Abstract

In earlier papers in this series we determined the intrinsic stellar disc kinematics of 15 intermediate- to late-type edge-on spiral galaxies using a dynamical modelling technique. The sample covers a substantial range in maximum rotation velocity and deprojected face-on surface brightness, and contains seven spirals with either a boxy or peanut-shaped bulge. Here we discuss the structural, kinematical and dynamical properties. From the photometry we find that intrinsically more flattened discs tend to have a lower face-on central surface brightness and a larger dynamical 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 modelling that at least 12 discs are submaximal. The average disc contributes 53 ± 4 per cent to the observed rotation at 2.2 disc scalelengths (hR), with a 1σ scatter of 15 per cent. 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 2.2hR the submaximal nature of discs is independent of barredness. The possibility remains that very high surface brightness discs are maximal, as these discs are underrepresented in our sample. We confirm that the radial stellar disc velocity dispersion is related to the galaxy maximum rotational velocity. The scatter in this σ versus vmax 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 and can generally be reproduced by the simple collapse theory for disc galaxy formation. Finally, the disc mass Tully–Fisher relation is offset from the maximum-disc scaled stellar mass Tully–Fisher relation of the Ursa Major cluster. This offset, −0.3 dex in mass, is naturally explained if the discs of the Ursa Major cluster spirals are submaximal.