A 500 Parsec Halo Surrounding the Galactic Globular NGC 1851

TL;DR

This paper reports the discovery of a symmetric, extensive halo around the globular cluster NGC 1851, which challenges existing models of cluster evolution and suggests complex formation or interaction histories.

Contribution

It provides the first detailed observation of a large, symmetric halo around NGC 1851, highlighting features unexplained by current globular cluster evolution models.

Findings

Halo extends from 41 pc to over 250 pc

Halo contains about 0.1% of cluster's mass

No evidence of tidal tails found

Abstract

Using imaging that shows four magnitudes 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 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.