On the spiral structure of our Galaxy

It is shown that the properties of the spiral wave in the Galaxy are determined by the mass distribution of its flat subsystem rather than by the full mass distribution. Then it turns out that better agreement with the observed spiral pattern is provided by the 'long' waves in contrast to the 'short' waves in the theory of Lin et al. (1969). A hypothesis is suggested which associates the generating mechanism of spiral waves with a rotating bar of old stars in the center of the Galaxy. Depending on the velocity of the bar rotation and the bar length, different combinations of the normal wave pattern and bar-like structure may occur, which possibly explains the great variety of transition forms between normal and barred spirals. In the proposed theory the packet of spiral waves moves from the inner Lindblad resonance outward and could be permanently maintained by the 'generator' in the center of the Galaxy. Therefore, the difficulty associated with the rapid obliteration of the packet (Toomre, 1969) does not arise.