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Willman 1 - a probable dwarf galaxy with an irregular kinematic distribution | | Beth Willman
; Marla Geha
; Jay Strader
; Louis E. Strigari
; Joshua D. Simon
; Evan Kirby
; Alex Warres
; | | Date: |
20 Jul 2010 | | Abstract: | 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 is a nearby, ultra-low luminosity Milky Way
companion. This object lies in a region of size-luminosity space (M_V ~ -2 mag,
d ~ 38 kpc, r_half ~ 20 pc) also occupied by the Galactic satellites Bo"otes
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 -12.8 +- 1.0 km/s, 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 an abundance spread in previous work. The significant spread in
the [Fe/H] of the two brightest Wil 1 red giant branch members ([Fe/H] = -1.65
+- 0.13 and -2.7 +- 0.15) supports the scenario that Wil 1 is a dwarf galaxy,
or the remnants thereof, rather than a star cluster. However, Wil 1’s innermost
stars move with radial velocities offset by 8 km/s from its outer stars,
suggesting that Wil 1 may not be in dynamical equilibrium. The combination of
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 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, in spite of the unusual features
in the Wil 1 kinematic distribution, present evidence indicates that this
object is, or at least once was, a dwarf galaxy. | | Source: | arXiv, 1007.3499 | | Services: | Forum | Review | PDF | Favorites | |
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