Planetary nebulae as kinematic tracers of galaxy stellar halos

TL;DR

This paper reviews how planetary nebulae serve as effective kinematic tracers for studying the stellar halos of galaxies, especially early-type ones, overcoming observational challenges.

Contribution

It summarizes recent advancements in using planetary nebulae to measure galaxy halo kinematics, highlighting new observational techniques and findings.

Findings

PNe enable halo kinematic measurements beyond 5 effective radii.

Recent surveys have provided kinematic data for dozens of galaxies.

PNe are valuable tracers in gas-poor early-type galaxies.

Abstract

The kinematic and dynamical properties of galaxy stellar halos are difficult to measure because of the faint surface brightness that characterizes these regions. Spiral galaxies can be probed using the radio HI emission; on the contrary, early-type galaxies contain less gas, therefore alternative kinematic tracers need to be used. Planetary nebulae (PNe) can be easily detected far out in the halo thanks to their bright emission lines. It is therefore possible to map the halo kinematics also in early-type galaxies, typically out to 5 effective radii or beyond. Thanks to the recent spectroscopic surveys targeting extra-galactic PNe, we can now rely on a few tens of galaxies where the kinematics of the stellar halos are measured. Here, I will review the main results obtained in this field in the last decades.