  NGC1851 is surrounded by a stellar component that extends more than ten times
beyond the tidal radius. Although the nature of this stellar structure is not
known, it has been suggested to be a sparse halo of stars or associated with a
stellar stream. We analyse the nature of this intriguing stellar component
surrounding NGC1851 by investigating its radial velocities and chemical
composition, in particular in comparison with those of the central cluster
analysed in a homogeneous manner. In total we observed 23 stars in the halo
with radial velocities consistent with NGC1851, and for 15 of them we infer
[Fe/H] abundances. Our results show that: (i) stars dynamically linked to
NGC1851 are present at least up to ~2.5 tidal radii, supporting the presence of
a halo of stars surrounding the cluster; (ii) apart from the NGC1851 radial
velocity-like stars, our observed velocity distribution agrees with that
expected from Galactic models, suggesting that no other sub-structure (such as
a stream) at different radial velocities is present in our field; (iii) the
chemical abundances for the s-process elements Sr and Ba are consistent with
the s-normal stars observed in NGC1851; (iv) all halo stars have metallicities,
and abundances for the other studied elements Ca, Mg and Cr, consistent with
those exhibited by the cluster. The complexity of the whole NGC1851
cluster+halo system may agree with the scenario of a tidally-disrupted dwarf
galaxy in which NGC1851 was originally embedded.
