Quarter-Turn Spirals in Galaxies

Abstract

Observations in the optical show that grand design spirals consist of a set of principle arms and characteristic near-circular extensions that can be described as quarter-turn spirals. Arguments are presented in favor of the idea that the latter set of spirals is caused by the response of the material of the disk to the gravitational potential of the main spiral arms. The peculiarities of the potential in the narrow transitional annulus between the regions of spiral and multipole behavior can explain basic characteristic features of the quarter-turn spirals (their angular length and small pitch angles).