A low surface brightness halo surrounding the globular cluster NGC 5694

TL;DR

This study discovers an extended stellar halo around globular cluster NGC 5694, revealing a smooth, extensive structure beyond the traditional tidal radius, with implications for understanding cluster outskirts.

Contribution

First detection of a low surface brightness halo around NGC 5694 using deep wide-field photometry, challenging existing models of cluster structure.

Findings

Halo extends up to 93 pc from the center.

No tidal tails detected within the observed field.

Halo contains about 3.5% of the cluster's total light.

Abstract

We report on the discovery of an extended stellar halo surrounding the distant Galactic globular cluster NGC 5694, based on new deep (V ~ 24.5) wide-field (24' * 20') photometry acquired with VIMOS at VLT. Stars with colour and magnitude consistent with the Main Sequence of the cluster are clearly identified out to r ~ 9'(~ 93 pc) from the cluster center, much beyond the tidal radius of the King model that best fits the inner profile (r_t=3.15'). We do not find a clear end of the structure within our field. The overall observed profile cannot be properly fitted with either a King (1966) model, an Elson et al. (1987) model, or a Wilson (1975) model; however it is very smooth and does not show any sign of the break near the tidal radius that is typically observed in stellar systems with tidal tails. The density map we derived does not show evidence of tidal tails, within the considered field. The extra-tidal component contains ~ 3.5% of the cluster light (mass) and has a surface density profile falling as ~ r^{-3.2}. The possible origin of the detected structure is discussed, as a clear-cut conclusion cannot be reached with the available data.