Recovering the intrinsic shape of early-type galaxies

TL;DR

This study demonstrates that combining photometric and stellar kinematic data allows for accurate recovery of the intrinsic shapes of early-type galaxies, especially when complex features are present, using realistic dynamical models.

Contribution

It introduces a method to recover the intrinsic shape of early-type galaxies from combined photometric and kinematic observations without assuming galaxy orientation.

Findings

Dynamical models can exclude triaxiality in axisymmetric galaxies.

Intrinsic shape recovery is limited for round or non-rotating galaxies without additional constraints.

Complex features like twists and decoupled components improve shape recovery accuracy.

Abstract

We investigate how well the intrinsic shape of early-type galaxies can be recovered when both photometric and two-dimensional stellar kinematic observations are available. We simulate these observations with galaxy models that are representative of observed oblate fast-rotator to triaxial slow-rotator early-type galaxies. By fitting realistic triaxial dynamical models to these simulated observations, we recover the intrinsic shape (and mass-to-light ratio), without making additional (ad-hoc) assumptions on the orientation. For (near) axisymmetric galaxies the dynamical modelling can strongly exclude triaxiality, but the regular kinematics do not further tighten the constraint on the intrinsic flattening significantly, so that the inclination is nearly unconstrained above the photometric lower limit even with two-dimensional stellar kinematics. Triaxial galaxies can have additional complexity in both the observed photometry and kinematics, such as twists and (central) kinematically decoupled components, which allows the intrinsic shape to be accurately recovered. For galaxies that are very round or show no significant rotation, recovery of the shape is degenerate, unless additional constraints such as from a thin disk are available.