DECam photometry reveals extra-tidal stars around the Milky Way globular cluster NGC 6864 (M75)

TL;DR

Using deep DECam photometry, the study detects an extended stellar envelope around the globular cluster NGC 6864, revealing tidal debris and suggesting a common mass loss pattern among Gaia-Enceladus associated clusters.

Contribution

First detection of an extra-tidal envelope around NGC 6864 using deep photometry, indicating tidal debris presence in a cluster previously thought to lack tails.

Findings

NGC 6864 has an extended stellar envelope and debris.

Most Gaia-Enceladus globular clusters show tidal tails.

Supports a common tidal disruption pattern among these clusters.

Abstract

Globular clusters are prone to lose stars while moving around the Milky Way. These stars escape the clusters and are distributed throughout extended envelopes or tidal tails. However, such extra-tidal structures are not observed in all globular clusters, and yet there is no structural or dynamical parameters that can predict their presence or absence. NGC\,6864 is an outer halo globular cluster with reported no observed tidal tails. We used Dark Energy Camera (DECam) photometry reaching $\sim$ 4 mag underneath its main sequence turnoff to confidently detect an extra-tidal envelope, and stellar debris spread across the cluster outskirts. These features emerged once robust field star filtering techniques were applied to the fainter end of the observed cluster main sequence. NGC\,6864 is associated to the {\it Gaia}-Enceladus dwarf galaxy, among others 28 globular clusters. Up-to-date, nearly 64$\%$ of them have been targeted looking for tidal tails and most of them have been confirmed to exhibit tidal tails. Thus, the present outcomes allow us to speculate on the possibility that {\it Gaia}-Enceladus globular clusters share a common pattern of mass loss by tidal disruption.