The age of the old Magellanic Cloud clusters -- II. NGC 1786, 1841 and 2210 as evidence for an old coeval population of LMC and galactic globular clusters

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Abstract
We present new, deep (V, V−I) colour-magnitude diagrams (CMDs) for the three old, metal-poor Magellanic Cloud (MC) clusters: NGC 1786, 1841 and 2210. The photometry reaches V ≃ 24 mag, yielding the first insight into the turn-off region and the upper main sequence of these clusters. On the basis of these new data, we found that the three clusters have quite similar CMDs, which closely resemble that of the metal-poor Galactic globular M68. Our main conclusion is that the most metal-poor clusters in the MC have not only similar metallicity to, but are also of age comparable with the metal-poor globulars in the Galaxy. Considering the observational uncertainties we estimate the relative age difference uncertainty is ~ 20 per cent (i.e. 3 Gyr). This evidence suggests an evolutionary scenario in which the production of globular clusters started in a similar way in the two galaxies, whilst more metal-rich clusters show evidence of a different evolution, as proved by the occurrence in the Large Magellanic Cloud (LMC) of clusters which are actually intermediate between the two galactic Oosterhoff types.

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