Abstract
The best-fitting two-dimensional plane within the three-dimensional space of spiral galaxy disc observables (rotational velocity υrot, central disc surface brightness μ0=-2.5 log I0 and disc scalelength h) has been constructed. Applying the three-dimensional bisector method of regression analysis to a sample of ∼100 spiral galaxy discs that span more than 4 mag arcsec−2 in central disc surface brightness yields (B band) and (R band). Contrary to popular belief, these results suggest that in the B band, the dynamical mass-to-light ratio (within four disc scalelengths) is largely independent of the surface brightness, varying as . Consistent results were obtained when the range of the analysis was truncated by excluding the low-surface-brightness galaxies. Previous claims that M/LBvaries withare shown to be misleading and/or caused by galaxy selection effects – not all low-surface-brightness disc galaxies are dark matter dominated. The situation is, however, different in the near-infrared where LK′∝υ4 and M/LK′ is shown to vary as . Theoretical studies of spiral galaxy discs should therefore not assume a constant M/L ratio within any given passband.
The B-band dynamical mass-to-light ratio (within four disc scalelengths) has no obvious correlation with (B-R) disc colour, while in the K′ band it varies as -1.25±0.28(B-R). Combining the present observational data with recent galaxy model predictions implies that the logarithm of the stellar-to-dynamical mass ratio is not a constant value, but increases as discs become redder, varying as 1.70±0.28(B-R).