On the physical size of the Milky Way globular cluster NGC 7089 (M 2)

TL;DR

This study analyzes the outer regions of the Milky Way globular cluster NGC 7089 using new observations and simulations, revealing an extended envelope and correlations between density profile slopes and cluster mass.

Contribution

It provides new observational evidence of an extended stellar envelope in NGC 7089 and explores the relationship between density profiles and cluster mass across multiple globular clusters.

Findings

Confirmed stars are found up to ~1 degree from the cluster center.

Observed density profiles agree well with N-body simulations.

Found a correlation between density slope and cluster mass in clusters with tidal tails.

Abstract

We study the outer regions of the Milky Way globular cluster NGC 7089 based on new Dark Energy Camera (DECam) observations. The resulting background cleaned stellar density profile reveals the existence of an extended envelope. We confirm previous results that cluster stars are found out to ~ 1deg from the cluster's centre, which is nearly three times the value of the most robust tidal radii estimations. We also used results from direct N-body simulations in order to compare with the observations. We found a fairly good agreement between the observed and numerically generated stellar density profiles. Because of the existence of gaps and substructures along globular cluster tidal tails, we closely examined the structure of the outer cluster region beyond the Jacobi radius. We extended the analysis to a sample of 35 globular clusters, 20 of them with observed tidal tails. We found that if the stellar density profile follows a power law ~ r**(-alpha), the alpha slope correlates with the globular cluster present mass, in the sense that, the more massive the globular cluster the smaller the alpha value. This trend is not found in globular clusters without observed tidal tails. The origin of such a phenomenon could be related, among other reasons, to the proposed so-called potential escapers or to the formation of globular clusters within dark matter mini-haloes.