Introduction

As shown by gravitational N-body simulations and observations of highly flattened giant galaxies including the Milky Way, the central parts of these systems at distances of, say, r ≲ 0.7-1 kpc from the center rotate slowly, and their local circular velocities of regular galactic rotation become less than (or comparable to) the residual (random) velocities. In such a thin, practically nonrotating (‘pressure-supported’) central disk, a typical star moves along the bending, perpendicular to the equatorial plane layer, under the action of two forces which act in opposite directions: the destabilizing centrifugal force, Fc, and the restoring gravitational attraction, Fg. Obviously, fierce instabilities of the buckling kind developing perpendicular to the plane may not be avoided if Fc > Fg. The latter condition is nothing else than the well-known condition of the so-called firehose electromagnetic instability in collisionless plasmas. The source of free energy in the instability is the intrinsic anisotropy of a velocity dispersion (‘temperature’).