A 500 Parsec Halo Surrounding the Galactic Globular NGC 1851

Using imaging that shows 4 mag of main-sequence stars, we have discovered that the Galactic globular cluster NGC 1851 is surrounded by a halo that is visible from the tidal radius of 700 arcsec (41 pc) to more than 4500 arcsec (> 250 pc). This halo is symmetric and falls in density as a power law of r(-1.24). It contains approximately 0.1% of the dynamical mass of NGC 1851. There is no evidence for tidal tails. Current models of globular cluster evolution do not explain this feature, although simulations of tidal influences on dwarf spheroidal galaxies qualitatively mimic these results. Given the state of published models, it is not possible to decide between creation of this halo from either isolated cluster evaporation or from tidal or disk shocking, or from destruction of a dwarf galaxy in which this object may have once been embedded.