A low surface brightness halo surrounding the globular cluster NGC 5694★

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 × 20arcmin2) photometry acquired with the VIMOS at the VLT. Stars with colour and magnitude consistent with the main sequence of the cluster are clearly identified out to r≃ 9arcmin(≃93pc) from the cluster centre, much beyond the tidal radius of the King model that best fits the inner profile (rt= 3.15arcmin). 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 model, an Elson et al. model or a Wilson 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 per cent 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.