An Outer Halo Object With Multiple Stellar Tails

Willman, B., Masjedi, M., Hogg, D. W., Dalcanton, J. J., Martinez-Delgado, D., Blanton, M., and West, A.
American Astronomical Society Meeting 207, #91.01; Bulletin of the American Astronomical Society, Vol. 37, p.1319


Willman 1 has a distance of ˜40 kpc and MV ˜ -2.5. This absolute magnitude makes it less luminous than all known Milky Way dwarf galaxies and all but the three smallest globular clusters. The nature of this object is ambiguous; Its physical size is over an order of magnitude larger than similarly faint globular clusters, placing it on the size-luminosity relationship followed by dwarf galaxies. One possibility is that Willman 1 is being tidally disrupted. We investigate this possibility with wide-field photometry extending 3 magnitudes fainter than the main-sequence turnoff. We find that the stellar distribution of Willman 1 extends beyond its likely tidal radius, showing that it is strongly affected by the tidal field of the Milky Way. The smoothed spatial distribution of Willman 1's stars also displays multi-directional tail-like features. Such prominent multi-directional features are unusual, particularly for a distant object. There is also preliminary evidence for mass segregation of stars as bright as 2 magnitudes below the main sequence turnoff. Detailed study of the stellar tails and mass segregation of Willman 1 will shed light on its ambiguous nature and will give us new insight on the process of tidal disruption and halo formation.