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NGC1851 is surrounded by a stellar component that extends more than ten timesbeyond the tidal radius. Although the nature of this stellar structure is notknown, it has been suggested to be a sparse halo of stars or associated with astellar stream. We analyse the nature of this intriguing stellar componentsurrounding NGC1851 by investigating its radial velocities and chemicalcomposition, in particular in comparison with those of the central clusteranalysed in a homogeneous manner. In total we observed 23 stars in the halowith radial velocities consistent with NGC1851, and for 15 of them we infer[Fe/H] abundances. Our results show that: (i) stars dynamically linked toNGC1851 are present at least up to ~2.5 tidal radii, supporting the presence ofa halo of stars surrounding the cluster; (ii) apart from the NGC1851 radialvelocity-like stars, our observed velocity distribution agrees with thatexpected from Galactic models, suggesting that no other sub-structure (such asa stream) at different radial velocities is present in our field; (iii) thechemical abundances for the s-process elements Sr and Ba are consistent withthe s-normal stars observed in NGC1851; (iv) all halo stars have metallicities,and abundances for the other studied elements Ca, Mg and Cr, consistent withthose exhibited by the cluster. The complexity of the whole NGC1851cluster+halo system may agree with the scenario of a tidally-disrupted dwarfgalaxy in which NGC1851 was originally embedded.

The halo+cluster system of the Galactic globular cluster NGC1851