Willman 1 - A probable dwarf galaxy with an irregular kinematic distribution

Abstract

We investigate the kinematic properties and stellar population of the Galactic satellite Willman 1 (Wil1) by combining Keck/DEIMOS spectroscopy with Kitt Peak National Observatory mosaic camera imaging. Wil1, 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 (MV ∼ -2mag, d ∼ 38kpc, r half ∼ 20pc) 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 Wil1. With a systemic velocity of v helio = -12.8 ∼ 1.0kms-1, Wil1 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. The significant spread in the [Fe/H] of the highly likely Wil1 red giant branch members ([Fe/H] = -1.73 ∼ 0.12 and -2.65 ∼ 0.12) supports the scenario that Wil1 is an ultra-low luminosity dwarf galaxy, or the remnants thereof, rather than a star cluster. However, Wil1's innermost stars move with radial velocities offset by 8kms-1 from its outer stars and have a velocity dispersion consistent with 0kms-1, suggesting that Wil1 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 Wil1. As a result, X-ray or gamma-ray observations of Wil1 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 Wil1 kinematic distribution, evidence indicates that this object is, or at least once was, a dwarf galaxy. © 2011. The American Astronomical Society. All rights reserved..