Evidence for multiple populations in the massive globular cluster NGC 2419 from deep uVI LBT photometry

We present accurate wide-field uVI photometry of the remote and massive Galactic globular cluster NGC 2419, aimed at searching for the u - V colour spread along the red giant branch (RGB) which is generally interpreted as the photometric signature of the presence of multiple populations in globular clusters. Focusing on the RGB stars in the magnitude range 19.8 ≤ V ≤ 22.0, we find that (i) the u - V, u - I and the V - I spreads about the RGB ridge line are significantly larger than that expected from observational errors, accurately quantified by means of extensive artificial stars experiments, (ii) the intrinsic colour spreads in u-V and u- I are larger than in V - I, and (iii) the stars lying to the blue of the RGB ridge line in u - V and u - I are significantly more concentrated towards the cluster centre than those lying to the red of that line. All the above observational facts can be interpreted in a scenario where a sizable fraction of cluster stars belong to a second generation heavily enriched in helium. Finally, we find that bright RGB stars (17.5 < V < 19.0) having [Mg/Fe] < 0.0 lie preferentially on the red side of the cluster RGB, while those having [Mg/Fe] > 0.0 lie preferentially on the blue side