Radially Extended Kinematics in the S0 Galaxy NGC 2768 from Planetary Nebulae, Globular Clusters and Starlight

TL;DR

This study compares planetary nebulae, globular clusters, and starlight velocities in NGC 2768 to explore the galaxy's extended kinematics, revealing distinct bulge and disk dynamics and suggesting a possible transformation from a late-type galaxy.

Contribution

It provides the first detailed comparison of multiple tracers out to four effective radii in an S0 galaxy, highlighting the kinematic differences between bulge and disk components.

Findings

Bulge PNe and GCs follow similar density profiles.

Disk PNe and starlight are distinct components.

Disk shows spiral-like rotation and declining velocity dispersion.

Abstract

There are only a few tracers available to probe the kinematics of individual early-type galaxies beyond one effective radius. Here we directly compare a sample of planetary nebulae (PNe), globular clusters (GCs) and galaxy starlight velocities out to ~4 effective radii, in the S0 galaxy NGC 2768. Using a bulge-to-disk decomposition of a K-band image we assign PNe and starlight to either the disk or the bulge. We show that the bulge PNe and bulge starlight follow the same radial density distribution as the red subpopulation of GCs, whereas the disk PNe and disk starlight are distinct components. We find good kinematic agreement between the three tracers to several effective radii (and with stellar data in the inner regions). Further support for the distinct nature of the two galaxy components come from our kinematic analysis. After separating the tracers into bulge and disk components we find the bulge to be a slowly rotating pressure-supported system, whereas the disk reveals a rapidly rising rotation curve with a declining velocity dispersion profile. The resulting V/sigma ratio for the disk resembles that of a spiral galaxy and hints at an origin for NGC 2768 as a transformed late-type galaxy. A two-component kinematic analysis for a sample of S0s will help to elucidate the nature of this class of galaxy.