Probing the 2-D kinematic structure of early-type galaxies out to 3 effective radii

TL;DR

This study introduces a new technique for mapping the 2-D stellar kinematic structure of early-type galaxy halos out to three effective radii, revealing complex rotation profiles and challenging traditional classifications.

Contribution

A novel method for measuring 2-D velocity moments in galaxy halos using multi-object spectroscopy, providing detailed kinematic maps out to large radii.

Findings

Velocity dispersion declines slowly beyond 1 effective radius.

Rotation profiles vary, with some galaxies showing constant, decreasing, or increasing rotation speeds.

Galaxy classifications based on inner regions may change when outer regions are considered.

Abstract

We detail an innovative new technique for measuring the 2-D velocity moments (rotation velocity, velocity dispersion and Gauss-Hermite coefficients h$_3$ and h$_4$) of the stellar populations of galaxy halos using spectra from Keck DEIMOS multi-object spectroscopic observations. The data are used to reconstruct 2-D rotation velocity maps. Here we present data for five nearby early-type galaxies to ~3 effective radii. We provide significant insights into the global kinematic structure of these galaxies, and challenge the accepted morphological classification in several cases. We show that between 1-3 effective radii the velocity dispersion declines very slowly, if at all, in all five galaxies. For the two galaxies with velocity dispersion profiles available from planetary nebulae data we find very good agreement with our stellar profiles. We find a variety of rotation profiles beyond 1 effective radius, i.e rotation speed remaining constant, decreasing \emph{and} increasing with radius. These results are of particular importance to studies which attempt to classify galaxies by their kinematic structure within one effective radius, such as the recent definition of fast- and slow- rotator classes by the SAURON project. Our data suggests that the rotator class may change when larger galacto-centric radii are probed. This has important implications for dynamical modeling of early-type galaxies. The data from this study are available on-line.