stable disks makes it possible to put constraints on its surface density: $$\label{Eq-cr-QToomre}
\sigma (R) = \frac{\kappa (R) c_r (R)}{3.36 G Q(R)},$$ and hence on a total disk mass therefore. It enables to obtain relative masses of disk and spheroidal components (bulge and dark halo) (Bottema (1993), Bottema (1997), Tyurina et al. (2001), Zasov et al. (2006)) and to estimate local thickness of stellar disk (see f.e. Khoperskov et al., 2010), which allows in turn to find a volume density of gas layer in disk plane if it exists in a galaxy (Zasov, Abramova (2006), Kasparova, Zasov (2008), Zasov, Abramova (2008)).

However the question of how far are the real disks from marginally stable state still remains open. There are two indirect evidences of closeness of stellar velocity dispersion to $(c_r)_{cr}$ for significant fraction of disk galaxies. First, disk mass-to-light ratios, which follow from the stability condition are in good agreement with the estimates based on stellar population evolution photometrical models (see e.g. Bottema (1997), Zasov et al. (2004)). Second, the assumption of disk marginal stability enables to explain a positive correlation of relative disks thicknesses and their relative masses within the optical borders (Zasov et al. (2002)). Note, however, 