SDSSJ1049+5103 , commonly known as Willman 1 , is an extremely low-luminosity Milky Way companion whose properties are intermediate between those of globular clusters and dwarf spheroidals .
In this paper , we present new , wide-field photometry extending 3 mag below the main sequence turnoff .
These data show that this object is old , moderately metal-poor , has a distance of 38 \pm 7 kpc and a half-light radius of r _ { 1 / 2 } = 21 \pm 7 pc , confirming previous estimates .
Willman 1 ’ s revised luminosity is M _ { V } = -2.5 mag , which is somewhat fainter than the previous estimate .
These new data show that the total spatial extent of Willman 1 exceeds its tidal radius for a range of assumptions about its total mass and its orbit , suggesting it is significantly affected by the tidal field of the Milky Way .
The spatial distribution of Willman 1 ’ s main sequence stars also shows prominent multi-directional stellar tails .
The tidal interactions causing these tail features may explain the large physical size of Willman 1 relative to low-luminosity globular clusters .
At a distance of 40 kpc , it is the most distant Galactic object yet known to display prominent tails , and is the only distant satellite to display multi-directional tails .
Although we can not at present determine the cause of this unusual morphology , preliminary comparisons between the morphology of Willman 1 and published simulations suggest that it may be near the apocenter of its orbit , or that it may have interacted with another halo object .
We find a significant difference between the luminosity functions of stars in the center and in the tails of Willman 1 , strongly suggesting mass segregation much like that seen in Palomar 5 .
Although Willman 1 has more pronounced tidal tails than most confirmed Milky Way dwarf galaxies , because of its very low stellar mass we can not at present rule out the possibility that it has a dark matter halo .