On the origin of the stellar halo and multiple stellar populations in the globular cluster NGC 1851

Abstract

We propose that the observed stellar halo around the globular cluster (GC) NGC 1851 is evidence of its formation in the central region of its defunct host dwarf galaxy. We numerically investigate the long-term dynamical evolution of a nucleated dwarf galaxy embedded in a massive dark matter halo under the strong tidal field of the Galaxy. The dwarf galaxy is assumed to have a stellar nucleus (or a nuclear star cluster) that could be the progenitor for NGC 1851. We find that although the dark matter halo and the stellar envelope of the host dwarf of NGC 1851 can be almost completely stripped during its orbital evolution around the Galaxy, a minor fraction of stars in the dwarf can remain trapped by the gravitational field of the nucleus. The stripped nucleus can be observed as NGC 1851 with no/little dark matter, whereas stars around the nucleus can be observed as a diffuse stellar halo around NGC 1851. The simulated stellar halo has a symmetric distribution with a power-law density slope of ~-2 and shows no tidal tails within ~200pc from NGC 1851. We show that two GCs can merge with each other to form a new nuclear GC embedded in field stars owing to the low stellar velocity dispersion of the host dwarf. This result makes no assumption on the ages and/or chemical abundances of the two merging GCs. Thus, the observed stellar halo and characteristic multiple stellar populations in NGC 1851 suggest that NGC 1851 could have formed initially in the central region of an ancient dwarf galaxy. We predict that the stellar halo of NGC 1851 may have at least three different stellar populations. We also suggest some Galactic GCs with diffuse haloes, such as NGC 1904 and 5694, could be formed in a similar way to NGC 1851. We discuss the importance of GC merging within dwarfs in the formation of multiple stellar populations with abundance spreads in heavy elements in some Galactic GCs, such as M22 and NGC 2419. We also discuss other possible scenarios for the formation of the stellar halo around NGC 1851.