The SAMI Galaxy Survey: rules of behaviour for spin-ellipticity radial tracks in galaxies

TL;DR

This study uses spin-ellipticity radial tracks from the SAMI Galaxy Survey to classify galaxy types and identify substructures, demonstrating the method's effectiveness in revealing features beyond the galaxy centers.

Contribution

It introduces a morpho-dynamical classification scheme based on spin-ellipticity radial tracks, enhancing galaxy substructure detection and understanding of galaxy dynamics.

Findings

Spin-ellipticity radial tracks identify substructures like embedded and counter-rotating discs.

Bars are more visible in images than in stellar kinematics.

Different galaxy types exhibit distinct radial track patterns.

Abstract

We study the behaviour of the spin-ellipticity radial tracks for 507 galaxies from the Sydney AAO Multi-object Integral Field (SAMI) Galaxy Survey with stellar kinematics out to $\geq1.5R_\text{e}$. We advocate for a morpho-dynamical classification of galaxies, relying on spatially-resolved photometric and kinematic data. We find the use of spin-ellipticity radial tracks is valuable in identifying substructures within a galaxy, including embedded and counter-rotating discs, that are easily missed in unilateral studies of the photometry alone. Conversely, bars are rarely apparent in the stellar kinematics but are readily identified on images. Consequently, we distinguish the spin-ellipticity radial tracks of seven morpho-dynamical types: elliptical, lenticular, early spiral, late spiral, barred spiral, embedded disc, and 2-sigma galaxies. The importance of probing beyond the inner radii of galaxies is highlighted by the characteristics of galactic features in the spin-ellipticity radial tracks present at larger radii. The density of information presented through spin-ellipticity radial tracks emphasises a clear advantage to representing galaxies as a track, rather than a single point, in spin-ellipticity parameter space.