Disk-to-halo mass ratio evaluations based on the numerical models of collisionless disks

We propose that the lower bound of the stellar radial velocity dispersion c_r of an equilibrium stellar disk is determined by the gravitational stability condition. We compared the estimates of stellar velocity dispersion at radii r>(1.5 - 2)R_0 (where R_0 is the photometric radial scalelength of a disk), found in the literature, with the minimal values of c_r necessary for the disk to be in a stable state, using the results of numerical simulations of 3D collisional disks. This approach enables to estimate an upper limit of the local surface density and (if R_0 is known) a total masses of a disk and a dark halo. We argue that the old stellar disks of spiral galaxies with active star formation usually have the velocity dispersions which are close to the expected marginal values. A rough values of disk-to-total mass ratios (within the fixed radius) are found for about twenty spiral galaxies. Unlike spirals, the disks of the ``red'' Sa--S0 galaxies are evidently ``overheated'': their radial dispersion of velocities at r ~ 2 R_0 exceeds significantly the marginal values for gravitational stability.