We investigate the kinematic properties and stellar population of the Galactic satellite Willman 1 ( Wil 1 ) by combining Keck/DEIMOS spectroscopy with KPNO mosaic camera imaging .
Wil 1 , also known as SDSS J1049+5103 , is a nearby , ultra-low luminosity Milky Way companion .
This object lies in a region of size-luminosity space ( M _ { V } \sim - 2 mag , d \sim 38 kpc , r _ { half } \sim 20 pc ) also occupied by the Galactic satellites Boötes II and Segue 1 and 2 , but no other known old stellar system .
We use kinematic and color-magnitude criteria to identify 45 stars as possible members of Wil 1 .
With a systemic velocity of v _ { helio } = -12.8 \pm 1.0 km s ^ { -1 } , Wil 1 stars have velocities similar to those of foreground Milky Way stars .
Informed by Monte-Carlo simulations , we identify 5 of the 45 candidate member stars as likely foreground contaminants , with a small number possibly remaining at faint apparent magnitudes .
These contaminants could have mimicked a large velocity dispersion and abundance spread in previous work .
We confirm a significant spread in the abundances of the likely Wil 1 red giant branch members ( [ Fe/H ] = - 1.73 \pm 0.12 and - 2.65 \pm 0.12 , [ Ca/Fe ] = - 0.4 \pm 0.18 and + 0.13 \pm 0.28 ) .
This spread supports the scenario that Wil 1 is an ultra-low luminosity dwarf galaxy rather than a star cluster .
Wil 1 ’ s innermost stars move with radial velocities offset by 8 km s ^ { -1 } from its outer stars and have a velocity dispersion consistent with 0 km s ^ { -1 } , suggesting that Wil 1 may not be in dynamical equilibrium .
The combination of the foreground contamination and unusual kinematic distribution make it difficult to robustly determine the dark matter mass of Wil 1 .
As a result , X-ray or gamma-ray observations of Wil 1 that attempt to constrain models of particle dark matter using an equilibrium mass model are strongly affected by the systematics in the observations presented here .
We conclude that , despite the unusual features in the Wil 1 kinematic distribution , evidence indicates that this object is , or at least once was , a dwarf galaxy .