The intricate link between galaxy dynamics and intrinsic shape (or why so-called prolate rotation is a misnomer)

TL;DR

This paper challenges the assumed link between galaxy dynamics and intrinsic shape, revealing that prolate rotation is a misnomer and highlighting limitations in current shape inference methods using IFS data.

Contribution

It demonstrates that the commonly assumed relationship between dynamical and intrinsic shape alignment does not hold in simulations, questioning previous interpretations of galaxy shapes.

Findings

The dynamical-intrinsic shape relationship does not hold in Illustris.

Prolate rotation is a misnomer based on current assumptions.

Current shape inference methods may be unreliable.

Abstract

Many recent integral integral field spectroscopy (IFS) survey teams have used stellar kinematic maps combined with imaging to statistically infer the underlying distributions of galaxy intrinsic shapes. With now several IFS samples at our disposal, the method, which was originally proposed by M. Franx and collaborators in 1991, is gaining in popularity, having been so far applied to ATLAS3D, SAMI, MANGA and MASSIVE. We present results showing that a commonly assumed relationship between dynamical and intrinsic shape alignment does not hold in Illustris, affecting our ability to recover accurate intrinsic shape distributions. A further implication is that so-called "prolate rotation", where the bulk of stars in prolate galaxies are thought to rotate around the projected major axis, is a misnomer.