Elliptical galaxies: rotationally distorted, after all

TL;DR

This paper analyzes elliptical galaxies using a model that relates their shape and rotation, revealing distinctions between slow and fast rotators and interpreting the Hubble sequence as a sequence of rotationally distorted equilibrium configurations.

Contribution

It introduces a framework connecting galaxy ellipticity, rotation, and anisotropy, and applies it to observational data to interpret galaxy classifications.

Findings

Slow rotators have low ellipticity and rotation parameters.

Fast rotators exhibit higher ellipticity and rotation.

Elliptical galaxies can be viewed as rotationally distorted equilibrium states.

Abstract

Different sequences of ellipsoids are represented on the ellipticity-rotation plane. The rotation parameter is defined as the ratio of kinetic energy related to the mean tangential equatorial velocity component to kinetic energy related to tangential equatorial component velocity dispersion and residual motions. Systems with isotropic stress tensor are considered as adjoint configurations to their counterparts with anisotropic stress tensor, different angular momentum, and equal remaining parameters. Both nonequilibrium figures and figures elongated by imaginary rotation are represented on the ellipticity-rotation plane. An application is made to a reduced sample of elliptical galaxies. The position on the ellipticity-rotation plane of both sample objects and related adjoint configurations with isotropic stress tensor is inferred from existing observations within the SAURON project. With a single exception, slow rotators are characterized by low ellipticities, low anisotropy parameters, and low rotation parameters, while the contrary holds for fast rotators. A possible interpretation of slow rotators as nonrotating at all and elongated due to negative anisotropy parameters, is exploited. Finally, the elliptical side of the Hubble sequence is interpreted as a sequence of equilibrium (adjoint) configurations where the ellipticity is an increasing function of the rotation parameter, slow rotators correspond to early classes and fast rotators to late classes. In this view, boundaries are rotationally distorted regardless of angular momentum and stress tensor, where rotation has to be intended as due to additional kinetic energy of tangential equatorial velocity components, with respect to spherical configurations with isotropic stress tensor.