The surroundings of the Milky Way globular cluster NGC 6809

TL;DR

This study uses deep DECam observations to analyze the outer regions of the Milky Way globular cluster NGC 6809, revealing faint stellar excesses and discussing the absence of tidal tails in similar clusters.

Contribution

It provides the first detailed stellar density map of NGC 6809's outskirts and explores the impact of orbital chaos on tidal tail formation.

Findings

Detected faint stellar excesses at the cluster's outskirts.

No tidal tails observed in NGC 6809 or similar clusters within 5.5 kpc.

Chaotic orbits may reduce the likelihood of tidal tail formation.

Abstract

We study the outer regions of the Milky Way globular cluster NGC6809 based on Dark Energy Camera (DECam) observations, which reach nearly 6 mag below the cluster main sequence (MS) turnoff. In order to unveil its fainter outermost structure, we built stellar density maps using cluster MS stars, once the contamination of field stars was removed from the cluster color-magnitude diagram. We found that only the resulting stellar density map for the lightest stars exhibits some excesses of stars at opposite sides from the cluster centre that diminish soon thereafter at ~ 0.32 deg. Studied globular clusters with apogalactic distances smaller than that of NGC6809 (5.5 kpc) do not have observed tidal tails. The lack of detection of tidal tails in the studied inner globular cluster sample could be due to the reduced diffusion time of tidal tails by the kinematically chaotic nature of the orbits of these globular clusters, thus shortening the time interval during which the tidal tails can be detected. Further investigations with an enlarged cluster sample are needed to confirm whether chaotic and non-chaotic orbits are responsible for the existence of globular clusters with tidal tails and those with extra-tidal features that are different from tidal tails or without any signatures of extended stellar density profiles.