Formation of the spiral structure in SB galaxies

We investigate regions near the ends of the principal spiral arms in SB gal axies, where the non-axially symmetric part of the gravitational potential rapidly transforms to its asymptotic quadrupole form. The galactic disk res ponds to this transformation of the potential by forming nearly circular sp irals with an angular extent of the order of pi /2 (quarter-turn spirals). We consider the resonance mechanism for the formation of principal spirals. Expressions are derived for the resonance responses of disks with circular and nearly circular stellar orbits. (C) 2001 MAIK "Nauka/Interperiodica".