The SLUGGS Survey: stellar kinematics, kinemetry and trends at large radii in 25 early-type galaxies

TL;DR

This study uses extensive stellar kinematic data from 25 early-type galaxies to investigate their outer regions, revealing significant radial changes in rotation and shape, and comparing observations with galaxy formation models.

Contribution

The paper provides detailed kinematic maps and profiles out to large radii, confirming radial modulations and testing galaxy shape assumptions, thus advancing understanding of galaxy outskirts in the two-phase formation scenario.

Findings

Radial modulations in galaxy rotation support vary with radius.

Outer galaxy shapes are consistent with some triaxiality, especially in fast rotators.

Outer angular momentum aligns with model predictions, but radial variations are smaller than expected.

Abstract

Due to longer dynamical timescales, the outskirts of early-type galaxies retain the footprint of their formation and assembly. Under the popular two-phase galaxy formation scenario, an initial in-situ phase of star formation is followed by minor merging and accretion of ex-situ stars leading to the expectation of observable transitions in the kinematics and stellar populations on large scales. However, observing the faint galactic outskirts is challenging, often leaving the transition unexplored. The large scale, spatially-resolved stellar kinematic data from the SAGES Legacy Unifying Galaxies and GlobularS (SLUGGS) survey are ideal for detecting kinematic transitions. We present kinematic maps out to 2.6 effective radii on average, kinemetry profiles, measurement of kinematic twists and misalignments, and the average outer intrinsic shape of 25 SLUGGS galaxies. We find good overall agreement in the kinematic maps and kinemetry radial profiles with literature. We are able to confirm significant radial modulations in rotational versus pressure support of galaxies with radius so that the central and outer rotational properties may be quite different. We also test the suggestion that galaxies may be more triaxial in their outskirts and find that while fast rotating galaxies were already shown to be axisymmetric in their inner regions, we are unable to rule out triaxiality in their outskirts. We compare our derived outer kinematic information to model predictions from a two-phase galaxy formation scenario. We find that the theoretical range of local outer angular momentum agrees well with our observations, but that radial modulations are much smaller than predicted.