Part III. Formation of Spiral Arms and Dynamical Model of Barred Galaxies

Publisher Website
Google Scholar
Abstract
This paper deals with the dynamical model of gas in spiral and barred galaxies. Theories that have been proposed to account for the formation of spiral arms are briefly reviewed. A new model is then proposed in which both gravitational and magnetic fields are taken into consideration. In Chapter I, solutions of the basic equations for the model are obtained by using a parameter which measures the ratio of magnetic to gravitational potential energy and by making a perturbation expansion in this quantity. It is found that for appropriate initial magnetic fields, a spiral arm can be formed as a locus of the points of maximum density of gas and the symmetry of spiral arms can be explained. Noncircular motions of gas, outflow and lag in rotational velocity of gas are also discussed. The time scale for the arm formation as well as for the growth of magnetic induction in the arm is found to be of the order 2 × 108 years. In Chapter II, several consequences derived from the theory are discussed. It is shown that, if a turnover point exists in the galactic rotation curve, the ring-structure as observed in some galaxies can be formed around this point, superposed on the spiral structure. To reconcile the disparity between the time scale of spiral arms and the age of galaxies, a hypothesis is made that a kind of stationarity may be established in the galactic plane, once the spiral arm is formed with the degree of winding as observed in actual galaxies, that is, angular momentum and magnetic field intensity will remain unchanged for a larger time interval than the above time scale. To test this we assume that in our Galaxy, there exists the region of instability for circular orbit inside the 3 kpc-arm that works like a pump through which mass and angular momentum of the halo flow into the disk. The supply of angular momentum by this would be sufficient of drive a large scale galactic circulation of gas, but the arguments are not conclusive. In Chapter III, a dynamical model of barred galaxies is described. The possibilities are confirmed of the outflow of gas along the bar and of the subsequent formation of trailing spiral arms at the diametral-opposite ends of the bar. It is suggested that the θ-shaped barred galaxies may be transient features appearing at a late stage of the evolution sequence. The maximum life-time of barred galaxies will lie between 1 ∼ 4 × 109 years according to their size and the amount of gas passing through the bar per unit time.

This publication has 0 references indexed in Scilit: