The properties of unstable large-scale hydrodynamic (HD) modes of oscillations in the gaseous disk of a flat galaxy with a rotation curve having a velocity jump in the inner region of the disk are numerically investigated. It is shown that some of these modes can form a regular pseudo-ring, and others can generate a spiral structure in a model galactic disk, with the rotation curve that can approximate the observed rotation curves in nearby massive spiral galaxies (such as M31, M81). The

more »

... eristic time of formation of the regular structure turns out to be . (1 − 2) GY, the corotation is located in the region of the velocity jump (on the radius∼ 2 − 4 kpc). The properties of the generated densitywaves are determined by the parameters of the velocity jumpon the rotation curve (relative amplitude and steepness of the velocity profile decrease), as well as the temperature (velocity dispersion) of the gas in the disk. For the first time, it was investigated how the global HD modes and the spiral structures formed by them will change in the case of the presence of a jump in the profile of the equilibrium surface density. This model profile was set in the form of a slightly smeared falling density step. Main results were obtained for two-arm m = 2 azimuthal harmonic.