Gas kinetics of galactic disks explains rotation curves of S-type galaxies without a need for dark matter

Abstract

In this paper, I show that the generally accepted methods of classical mechanics are not applicable for calculating the outer parts of the rotation curves (RCs) of galaxies, where the influence of collisions on gas dynamics becomes dominant. In addition, the hydrodynamic approach cannot be used for this purpose due to an extreme rarefaction of the gas. I develop a new approach for describing the gas dynamics in outer regions of galactic disks, where the gas dynamics is determined mainly by collisions. Equations (free from restrictions imposed on hydrodynamics) are obtained that describe the dynamics of rarefied gas. The resulting equation [Formula: see text] (14) relates two quantities: the tangential velocity of the gas as a function of the distance from the center of a galaxy (RC) and the radial distribution of the gas density. It is shown that if the physical properties of the rarefied gas are properly taken into account, then dark matter (DM) is not required, and the “nonphysical” (non-Keplerian) RCs of the outer parts of the galactic disks are tailwinds that can be described within the framework of conventional gas kinetics. To illustrate the correctness of the obtained model, two galaxies with flat RCs (NGC7331 and NGC3198) are considered. From the observed RCs, using Eq. ( 14 ), the radial densities of the gas are calculated. An excellent agreement was obtained between the calculated gas densities and their observed values, which is a serious argument in favor of the developed model. Thus, the nonphysical RCs of spiral galaxies represent the tailwinds of gas, the dynamics of which is naturally described by the kinetic equation without involving the concept of DM. Total masses of two galaxies NGC7331 and NGC3198 have been calculated. Implications for cosmology are discussed.