Mass‐to‐light ratios from the fundamental plane of spiral galaxy discs

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 =‐2.5 log  I 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 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 / L B varies with 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 . 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 ) .