Spiral galaxies with non-typical mass-to-light ratios

Total mass-to-light ratio M/L _B for S0 — Irr galaxies, whereM is the dynamical mass within the optical radius R ₂₅ (corresponding to 25 ^m /sq. arcsec in the B band), increases systematically with (B-V)₀ color, but slower than that is predicted by stellar population evolution models without dark halo. It shows that the mean ratio between dark halo and stellar masses is higher for more “blue“ galaxies. However some galaxies don’t follow this general trend. The properties of galaxies with extremely high and extremely low values of M/L _B ratios are compared, and different factors, accounting for the extremes, are analyzed. The conclusion is that in some cases too high or too low M/L _B ratios are associated with observational errors, in other cases—with non-typical dark halo mass fraction, and in some cases — with peculiarities of disc stellar population. Particularly, discs of some galaxies with low M/L _B ratios turn out to be unusually “light” for their luminosity and colors, which indicates a substantial deficit of low mass stars as the most probable cause of low M/L _B .