Mass-to-light ratios from the fundamental plane of spiral galaxy disks

The best-fitting 2-dimensional plane within the 3-dimensional space of spiral galaxy disk observables (rotational velocity vrot, central disk surface brightness μ0 = −2.5 log I0, and disk scale-length h) has been constructed. Applying a threedimensional bisector method of regression analysis to a sample of ∼100 spiral galaxy disks that span more than four mag arcsec in central disk surface brightness yields vrot ∝ I 0.50±0.05 0 h (B-band) and vrot ∝ I 0.43±0.04 0 h (R-band). Contrary to popular belief, these results suggest that in the B-band, the dynamical massto-light ratio (within 4 disk scale-lengths) is largely independent of surface brightness, varying as I 0 h. Consistent results were obtained when the expanse of the analysis was truncated by excluding the low surface brightness galaxies. Previous claims that M/LB varies with I −1/2 0 are shown to be misleading and/or due to galaxy selection effects. Not all low-surface-brightness disk galaxies are dark matter dominated. The situation is however different in the near-infrared where LK′ ∝ v 4 and M/LK′ is shown to vary as I −1/2 0 . Theoretical studies of spiral galaxy disks should not assume a constant M/L ratio within any given passband. The B-band dynamical mass-to-light ratio (within 4 disk scale-lengths) has no obvious correlation with (B − R) disk 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 disks become redder, varying as 1.70±0.28(B −R).