Matthew G. Walker

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We apply the Jeans equation to estimate masses for eight of the brightest dwarf spheroidal (dSph) galaxies. For Fornax, the dSph with the largest kinematic data set, we obtain a model-independent constraint on the maximum-circular velocity, V max = 20 +4 −3 km s −1. Although we obtain only lower-limits of V max 10 km s −1 for the remaining dSphs, we find(More)
We present stellar velocity dispersion profiles for seven Milky Way dwarf spheroidal (dSph) satellite galaxies. We have measured 8394 line-of-sight velocities (±2.5 km s −1) for 6804 stars from high-resolution spectra obtained at the Magellan and MMT telescopes. We combine these new data with previously published velocities to obtain the largest available(More)
  • Matthew G Walker, Mario Mateo, Edward W Olszewski, Rebecca A Bernstein, Xiao Wang, Michael Woodroofe
  • 2006
We present new radial velocity results for 178 stars in the Fornax dwarf spheroidal galaxy, of which at least 158 are probable Fornax members. We combine with previously published data to obtain a radial velocity sample with 209 stars, of which at least 179 are probable Fornax members. We detect the hint of rotation about an axis near Fornax's morphological(More)
The Milky Way has at least twenty-three known satellite galaxies that shine with luminosities ranging from about a thousand to a billion times that of the Sun. Half of these galaxies were discovered in the past few years in the Sloan Digital Sky Survey, and they are among the least luminous galaxies in the known Universe. A determination of the mass of(More)
  • Xiao Wang, Michael Woodroofe, Matthew G Walker, Mario Mateo, Edward Olszewski
  • 2005
Thanks to instrumental advances, new, very large kinematic datasets for nearby dwarf spheroidal (dSph) galaxies are on the horizon. A key aim of these datasets is to help determine the distribution of dark matter in these galaxies. Past analyses have generally relied on specific dynamical models or highly restrictive dynamical assumptions. We describe a(More)
We present spectroscopic data for individual stars observed from 2004 March through 2008 August as part of our Michigan/MIKE Fiber System (MMFS) survey of four dwarf spheroidal (dSph) galaxies: Carina, Fornax, Sculptor and Sextans. Using MMFS at the Magellan/Clay Telescope at Las Campanas Observatory , we have acquired 8855 spectra from 7103 red giant(More)
We develop an algorithm for estimating parameters of a distribution sampled with contamination. We employ a statistical technique known as " expectation maximization " (EM). Given models for both member and contaminant populations, the EM algorithm iteratively evaluates the membership probability of each discrete data point, then uses those probabilities to(More)
This paper presents the results and experiences of adapting and improving the Many-Task Computing (MTC) framework Kestrel for use with bag of tasks applications and the STAR experiment in particular. Kestrel is a lightweight, highly available job scheduling framework for Virtual Organization Clusters (VOCs) constructed in the cloud. Kestrel uses the(More)
We present a search for γ-ray emission from the direction of the newly discovered dwarf galaxy Reticulum II. Using Fermi-LAT Collaboration data, we detect a signal that exceeds expected backgrounds between ∼2-10  GeV and is consistent with annihilation of dark matter for particle masses less than a few ×10^{2}  GeV. Modeling the background as a Poisson(More)
The transverse motions of nearby dwarf spheroidal (dSph) galaxies contribute line-of-sight components that increase with angular distance from the dSph centers, inducing detectable gradients in stellar redshift. In the absence of an intrinsic velocity gradient (e.g., due to rotation or streaming), an observed gradient in the heliocentric rest frame (HRF)(More)