The Globular Cluster System of NGC 6822

TL;DR

This study analyzes the globular cluster system of NGC 6822 using photometry and spectroscopy, revealing old ages, metallicity range, weak rotation, and a high dark matter content, leading to a mass estimate of 3-4 billion solar masses.

Contribution

It provides the first combined optical and near-IR photometry with spectroscopic data for NGC 6822's GCs, and estimates the galaxy's mass and dark matter dominance.

Findings

GCs are old, ~9 Gyr or older.

GC metallicities range from -1.6 to -0.4.

NGC 6822's mass within 11 kpc is (3-4)×10^9 solar masses.

Abstract

We present a comprehensive analysis of the globular cluster (GC) system of the Local Group dwarf irregular galaxy NGC 6822. Our study is based on homogeneous optical and near-IR photometry, as well as long-slit spectroscopic observations which are used to determine new radial velocities for 6 GCs, two of which had no previous spectroscopic information. We construct optical-near IR colour-colour diagrams and through comparison to simple stellar population models infer that the GCs have old ages consistent with being 9 Gyr or older, while their metallicities are in the range between -1.6 < [Fe/H] < -0.4. We conduct a kinematic analysis of the GC population and find tentative evidence for weak net rotation of the GC system, in the same sense as that exhibited by the underlying spheroid. The most likely amplitude of rotation is ~10 km/s, approximately half the magnitude of the observed velocity dispersion. Finally, we use the GCs to estimate the dynamical mass of NGC 6822 within 11 kpc and we formally find it to be in the range between (3-4)10^9 Msun. This implies an overall mass-to-light ratio in the range of ~ 30-40 and indicates that NGC 6822 is highly dark matter dominated. The mass and the corresponding mass-to-light ratio estimates are affected by various additional systematic effects due to limitations of the data and the model that are not necessary reflected in the formal uncertainties.