Separatrix crossing and the enigma of NGC 4550

Abstract

Whenever elliptical disks become circular, or triaxial halos become axisymmetric, then stars moving on box orbits escape from the confining azimuthal potential well -- crossing the separatrix -- to move onto tube orbits. In nonrotating disks, there are as many boxes with instantaneous clockwise azimuthal motion as counterclockwise. So they are scattered half onto clockwise streaming tube orbits, half onto counterclockwise. This is a natural mechanism for building identical counterstreaming galactic disks with the same ages and scale lengths, like NGC 4550. Two possible dynamical histories are presented for this galaxy. They are both based on separatrix crossing but differ in the hotness of the resulting disks.