  We propose that the observed stellar halo around the globular cluster (GC)
NGC 1851 is evidence for 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.
