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Post Info TOPIC: Willman 1
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Title: Willman 1 - a probable dwarf galaxy with an irregular kinematic distribution
Authors: Beth Willman, Marla Geha, Jay Strader, Louis E. Strigari, Joshua D. Simon, Evan Kirby, Alex Warres

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 Bootes II and Segue 1 and 2, but no other known old stellar system. We use kinematic and colour-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.

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Title: Willman 1 in X-rays: Can you tell a dwarf galaxy from a globular cluster?
Authors: N. Mirabal, D. Nieto (Universidad Complutense de Madrid)

We present an analysis of a deep archival Chandra observation of Willman 1, an object suspected to straddle the line of what constitutes a dwarf galaxy and an extreme globular cluster. Our main goal is to examine potential observational signatures in X-rays that might distinguish its true identity either through an unusual point source population or based on the existence of prominent diffuse emission in its core. We identify a total of 26 sources within the central 5 arcminutes to a limiting 0.5-2.0 keV X-ray flux of 6 x 10^{-16} ergs/cm^{2}/s. While some of these sources could be formal members of Willman 1, we find no outstanding evidence for either an unusual population of bright X-ray sources or a densely populated clustercore. In fact, the entire X-ray population could be explained by background active galactic nuclei and/or foreground stars unrelated to Willman 1. As a result, there is no substantial evidence in X-rays to argue against a dwarf galaxy classification for Willman 1 down to current observational limits. This result enhances the qualifications of Willman 1 as an ideal target for indirect dark matter searches. Accordingly, we derive upper limits for a possible sterile neutrino signature with a mass of 1.6-16.0 keV and finish with a discussion of previous measurements.

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