2002MNRAS.334..721G

The best-fitting two-dimensional plane within the three-dimensional space of spiral galaxy disc observables (rotational velocity v _rot, central disc surface brightness µ ₀ =-2.5logI ₀ 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 4magarcsec^–2 in central disc surface brightness yields {formmu6} (B band) and {formmu7} (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 {formmu8}. Consistent results were obtained when the range of the analysis was truncated by excluding the low-surface-brightness galaxies. Previous claims that M/L _ B _ varies with {formmu9}are 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 L _ K '_ ∝v ⁴ and M/L _ K '_ is shown to vary as {formmu12}. 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).