The Outer Envelopes of Globular Clusters. II. NGC 1851, NGC 5824 and NGC 1261

TL;DR

This study investigates the extended stellar envelopes around three Milky Way globular clusters, revealing diverse sizes and profiles, and discusses their possible origins, including stripped dwarf galaxy nuclei or dynamical evolution.

Contribution

First detailed analysis of stellar envelopes around NGC 1261, NGC 1851, and NGC 5824 using wide-field photometry, highlighting their sizes, shapes, and density profiles.

Findings

NGC 1851 has a large, elliptical envelope at least 240 pc in radius.

NGC 5824 is highly extended with a size similar to NGC 1851's envelope.

NGC 1261 has a smaller, steeper envelope around 105 pc in radius.

Abstract

We present a second set of results from a wide-field photometric survey of the environs of Milky Way globular clusters. The clusters studied are NGC 1261, NGC 1851 and NGC 5824: all have data from DECam on the Blanco 4m telescope. NGC 5824 also has data from the Magellan Clay telescope with MegaCam. We confirm the existence of a large diffuse stellar envelope surrounding NGC 1851 of size at least 240 pc in radius. The radial density profile of the envelope follows a power-law decline with index $\gamma = -1.5 \pm 0.2$ and the projected shape is slightly elliptical. For NGC 5824 there is no strong detection of a diffuse stellar envelope, but we find the cluster is remarkably extended and is similar in size (at least 230 pc in radius) to the envelope of NGC 1851. A stellar envelope is also revealed around NGC 1261. However, it is notably smaller in size with radius $\sim$105 pc. The radial density profile of the envelope is also much steeper with $\gamma = -3.8 \pm 0.2$. We discuss the possible nature of the diffuse stellar envelopes, but are unable to draw definitive conclusions based on the current data. NGC 1851, and potentially NGC 5824, could be stripped dwarf galaxy nuclei, akin to the cases of $\omega$ Cen, M54 and M2. On the other hand, the different characteristics of the NGC 1261 envelope suggest that it may be the product of dynamical evolution of the cluster.