Assistant Professor, Physics
EducationPh.D., University of Michigan
ResearchThere are two ways to measure the mass of a typical galaxy: 1) count the number of stars and multiply by the mass of a typical star, and 2) measure the orbital positions and speeds of those same stars, plug the numbers into the law of gravity (which relates mass to position and speed), and solve for mass. The problem is that the two answers don't agree. For nearly every galaxy that has been observed, the 'luminous mass' obtained by counting visible objects is smaller than the 'dynamical mass' inferred by applying (Newtonian) gravity to the motions of visible objects. The discrepancy between luminous and dynamical mass is called 'dark matter'---mass that is detected only via gravitation---and seems to represent ~85% of the mass in the Universe.
I study the astrophysical properties of dark matter, thus far via optical imaging, spectroscopy and dynamical modelling of the 'dwarf' galaxies that surround the Milky Way and neighboring Andromeda. The dwarf galaxies include the oldest, smallest and 'darkest' (i.e., composed almost entirely of dark matter) galaxies known, and currently represent the smallest physical scales (sizes of ~ 100 light years, speeds of a few kilometers per second, masses of ~100,000 Suns) that are associated empirically with dark matter. If dark matter is made from some kind of new fundamental particle, then the manner in which dark matter can form 'clumps' at such small scales can help to decide among various ideas about the properties of that particle. By measuring the spatial distribution of dark matter in dwarf galaxies, I aim to help figure out what the dark matter actually is. Thus my research has wandered into the intersection of dynamics, cosmology and particle physics.
For this work I use some of the world's largest optical telescopes, including the 6.5-meter Magellan telescopes at Las Campanas Observatory in Chile, the 6.5-meter MMT at Mt. Hopkins, Arizona, and the 8.2-meter Very Large Telescope at Cerro Paranal in Chile. I am also a member of the Sloan Digital Sky Survey IV collaboration.
- Alex Geringer-Sameth, Matthew G. Walker, Savvas M. Koushiappas, Sergey E. Koposov, Vasily Belokurov, Gabriel Torrealba, N. Wyn Evans, Indication of Gamma-Ray Emission from the Newly Discovered Dwarf Galaxy Reticulum II, Physical Review Letters 115, 81101 (2015)
- M. G. Walker, E. W. Olszewski, M. Mateo, Bayesian analysis of resolved stellar spectra: application to MMT/Hectochelle observations of the Draco dwarf spheroidal, Monthly Notices of the Royal Astronomical Society 448, 2717 (2015)
- Matthew G. Walker, Abraham Loeb, Is the universe simpler than ΛCDM?, Contemporary Physics 55, 198 (2014)
- C. F. P. Laporte, M. G. Walker, J. Penarrubia, Measuring the slopes of mass profiles for dwarf spheroidals in triaxial cold dark matter potentials, Monthly Notices of the Royal Astronomical Society: Letters 433, L54 (2013)
- J. Zavala, M. Vogelsberger, M. G. Walker, Constraining self-interacting dark matter with the Milky Way's dwarf spheroidals, Monthly Notices of the Royal Astronomical Society: Letters 431, L20 (2013)
- Jorge Peñarrubia, Sergey E. Koposov, Matthew G. Walker, A statistical method for measuring the galactic potential and testing gravity with cold tidal streams, The Astrophysical Journal 760, 2 (2012)
- Jorge Peñarrubia, Andrew Pontzen, Matthew G. Walker, Sergey E. Koposov, The coupling between the core/cusp and missing satellite problems, The Astrophysical Journal 759, L42 (2012)
- Matthew G. Walker, Jorge Peñarrubia, A method for measuring (slopes of) the mass profiles of dwarf spheroidal galaxies, The Astrophysical Journal 742, 20 (2011)
- Matthew Walker, Dark Matter in the Galactic Dwarf Spheroidal Satellites, Planets, Stars and Stellar Systems, Volume 5: Galactic Structure and Stellar Populations , Springer-Verlag, p. 1039 (2013)
- M. G. Walker, C. Combet, J. A. Hinton, D. Maurin, M. I. Wilkinson, Dark matter in the classical dwarf spheroidal galaxies: a robust constraint on the astrophysical factor for γ-ray flux calculations, The Astrophysical Journal 733, L46 (2011)
- Matthew G. Walker, Stacy S. McGaugh, Mario Mateo, Edward W. Olszewski, Rachel Kuzio de Naray, Comparing the dark matter halos of spiral, low surface brightness, and dwarf spheroidal galaxies, The Astrophysical Journal 717, L87 (2010)
- Matthew G. Walker, Mario Mateo, Edward W. Olszewski, Jorge Peñarrubia, N. Wyn Evans, Gerard Gilmore, A universal mass profile for dwarf spheroidal galaxies?, The Astrophysical Journal 704, 1274 (2009)
- Matthew G. Walker, Mario Mateo, Edward W. Olszewski, Stellar velocities in the Carina, Fornax, Sculptor, and Sextans DSPH galaxies: Data from the Magellan/MMFS survey, The Astronomical Journal 137, 3100 (2009)
- Matthew G. Walker, Mario Mateo, Edward W. Olszewski, Systemic Proper Motions of Milky Way Satellites from Stellar Redshifts: The Carina, Fornax, Sculptor, and Sextans Dwarf Spheroidals, The Astrophysical Journal 688, L75 (2008)
- Matthew G. Walker, Mario Mateo, Edward W. Olszewski, Oleg Y. Gnedin, Xiao Wang, Bodhisattva Sen, Michael Woodroofe, Velocity Dispersion Profiles of Seven Dwarf Spheroidal Galaxies, The Astrophysical Journal 667, L53 (2007)