Bending Instabilities in Homogenous Oblate Spheroidal Galaxy Models

Abstract

Various authors have shown through computational modeling and analytic analysis that elliptical galaxies flatter than E7 will undergo dynamical instabilities collectively known as the firehose instability. Using N-body methods, Merritt & Hernquist followed the onset and evolution of the instability in prolate shell-orbit models. We have investigated the evolution of the instability and eventual quiet-system geometry of highly flattened axially symmetric oblate models. In this paper, we study a class of homogeneous spheroids first proposed by Polyachenko. We show that systems flatter than E7 are unstable, that they evolve to states which are less flat than E7 by distinct stages, and that, in many cases, the final isophotes are boxy ellipticals. Moreover, the final axial ratios are independent of the initial flattening.