The Shapes of Elliptical Galaxies with Central Density Cusps and Massive Dark Objects

Abstract

For a sample of 22 core ellipticals and 22 power law ellipticals, I examine the apparent axis ratios of isophotes as a function of the dynamical time at the isophote's mean radius. The two types of elliptical galaxy have a significantly different distribution of shapes in the region where the dynamical time is short (about 1 Myr); the core galaxies are rounder in this region. In the outer regions, the two types have indistinguishable shape distributions. I perform a more detailed analysis of the apparent and intrinsic shapes at two chosen locations: a inner region, where the dynamical time is 0.75 Myr, and an outer region, where the dynamical time is 50 Myr. The inner isophotes of core galaxies are consistent with their being randomly oriented oblate objects, with a peak intrinsic axis ratio of c/a = 0.86. The outer regions of core galaxies are inconsistent with their being randomly oriented oblate or prolate objects. The oblate hypothesis cannot be rejected for the inner and outer regions of power law ellipticals at the 90% confidence level. The observed shapes are consistent with the hypothesis that the inner regions of core galaxies are nearly spherical and nearly axisymmetric, perhaps as the result of the physical process which formed the core.