Gravitational stability and dynamical overheating of galactic stellar disks

Stellar velocity dispersion data at galactocentric distance of two disk radial scale lengths (R = 2h), available in the literature allowed us to determine the upper limits of disk local surface densities at a given R and (by extrapolation) total masses of disks proceeding from the marginal gravitational stability condition. A comparison of the obtained disk masses with the photometric estimates based on the stellar population models indicates the absence of strong dynamical overheating inmost spiral galaxies and hence the absence of significant major merging events, which were able to heat dynamically the inner parts of disks. The same conclusion is valid for some of S0 galaxies. However, a significant part of the latter possesses stellar velocity dispersion, which exceeds the threshold value needed for gravitational stability. Dynamically overheated disks occur both among paired and isolated galaxies. Disk to total mass ratios within R = 4h found for marginally stable disks in most cases lie in the range 0.5–0.8 with the absence of the clearly defined correlation of this ratio with color index or morphological type.