Intrinsic Shapes of Elliptical Galaxies

TL;DR

This paper investigates the intrinsic three-dimensional shapes of elliptical galaxies, analyzing observational data and statistical tests to understand their shape distribution and the role of projection effects and dynamical instabilities.

Contribution

It provides a comprehensive analysis of the shape distribution of elliptical galaxies, evaluating the oblate, prolate, and triaxial hypotheses with new statistical insights.

Findings

Few nearly spherical galaxies observed, challenging the oblate hypothesis.

Statistical tests are consistent with triaxial shapes.

Dynamical instabilities may prevent the existence of very flat or nearly spherical ellipticals.

Abstract

Tests for the intrinsic shape of the luminosity distribution in elliptical galaxies are discussed, with an emphasis on the uncertainties. Recent determinations of the ellipticity frequency function imply a paucity of nearly spherical galaxies, and may be inconsistent with the oblate hypothesis. Statistical tests based on the correlation of surface brightness, isophotal twisting, and minor axis rotation with ellipticity have so far not provided strong evidence in favor of the nearly oblate or nearly prolate hypothesis, but are at least qualitatively consistent with triaxiality. The possibility that the observed deviations of elliptical galaxy isophotes form ellipses are due to projection effects is evaluated. Dynamical instabilities may explain the absence of elliptical galaxies flatter than about E6, and my also play a role in the lack of nearly-spherical galaxies.