The Outer Envelopes of Globular Clusters. I. NGC 7089 (M2)

TL;DR

This study reveals that the globular cluster NGC 7089 (M2) is surrounded by a large, diffuse stellar envelope extending well beyond its nominal tidal radius, possibly indicating a complex origin related to galaxy accretion.

Contribution

First wide-field imaging survey of M2's periphery showing an extensive stellar envelope and discussing its potential origin as a stripped dwarf galaxy nucleus.

Findings

M2 has a diffuse stellar envelope extending to ~210 pc

The envelope's density follows a power law with index -2.2

M2 hosts diverse stellar populations with broad abundance dispersions

Abstract

We present the results of a wide-field imaging survey of the periphery of the Milky Way globular cluster NGC 7089 (M2). Data were obtained with MegaCam on the Magellan Clay Telescope, and the Dark Energy Camera on the Blanco Telescope. We find that M2 is embedded in a diffuse stellar envelope extending to a radial distance of at least $\sim 60^{\prime}$ ($\sim 210$ pc) -- five times the nominal tidal radius of the cluster. The envelope appears nearly circular in shape, has a radial density decline well described by a power law of index $\gamma = -2.2 \pm 0.2$, and contains approximately $1.6\%$ of the luminosity of the entire system. While the origin of the envelope cannot be robustly identified using the presently available data, the fact that M2 also hosts stellar populations exhibiting a broad dispersion in the abundances of both iron and a variety of neutron capture elements suggests that this object might plausibly constitute the stripped nucleus of a dwarf Galaxy that was long ago accreted and destroyed by the Milky Way.