A 500 PARSEC HALO SURROUNDING THE GALACTIC GLOBULAR NGC 1851

Abstract

Using imaging that shows four magnitudes of main sequence stars, we have\ndiscovered that the Galactic globular cluster NGC 1851 is surrounded by a halo\nthat is visible from the tidal radius of 700 arcsec (41 pc) to more than 4500\narcsec (>250 pc). This halo is symmetric and falls in density as a power law of\n$r^{-1.24}$. It contains approximately 0.1% of the dynamical mass of NGC 1851.\nThere is no evidence for tidal tails. Current models of globular cluster\nevolution do not explain this feature, although simulations of tidal influences\non dwarf spheroidal galaxies qualitatively mimic these results. Given the state\nof published models it is not possible to decide between creation of this halo\nfrom isolated cluster evaporation, or from tidal or disk shocking, or from\ndestruction of a dwarf galaxy in which this object may have once been embedded.\n