Dr. Robyn Sanderson is awarded an NSF Astronomy and Astrophysics Postdoctoral Fellowship to carry out a program of research and education at Columbia University and New York University. Outside the Galactic disk, where most stars including our Sun reside, most of our Galaxy's mass is in its roughly spherical halo of dark matter, invisible material only known to exist through its gravitational influence. Also out there are a handful of halo stars, including the scattered remains of smaller galaxies that entered the Milky Way's dark halo only to be ripped apart by its tidal forces. The structures left behind, called tidal streams, can be used to study the distribution of the Milky Way's dark matter. Each tidal stream is made up of stars that were once neighbors in the same small galaxy, and thus now follow similar paths as they orbit the Milky Way. This common origin gives us insight into the Galaxy's mass distribution, which determines the orbits of the stream's stars: only if we guess close to the true mass distribution do the computed orbits of stars in each stream cluster together around a common progenitor. Changes to the neighboring orbits of tidal stream stars could also provide evidence for smaller-scale dark matter structure: tiny dark matter clumps too small to hold galaxies, predicted by some theories of dark matter, would deflect the paths of stream stars in predictable ways. Dr. Sanderson will develop new tools to explore the structure of dark matter halos, particularly the Milky Way's, by studying their tidal streams. These tools use state-of-the-art methods, yet are based on fundamental concepts that kids learn in middle school: why stars shine, how we make and read maps, and how gravity works. To teach these core ideas and inspire middle schoolers' interest in science, Dr. Sanderson will develop portable science lessons for use in underserved New York City middle schools. The modules will use songs and movement the students produce themselves to introduce them to the "music of the spheres."
The work Dr. Sanderson will carry out as a fellow has three goals. First, she will expand a new method of inferring the Milky Way's mass distribution, which uses concepts borrowed from information theory to statistically analyze the clustering of halo stars in streams and thereby select the best fit for the Milky Way halo. This method will be applied to forthcoming data from the Gaia satellite, which will accurately measure complete positions and velocities for millions of stream stars in the Milky Way. Second, she will study the influence of small dark matter clumps on tidal streams, by predicting their effects on the orbits of the stream stars and determining which effects would be most diagnostic to observe. Finally, Dr. Sanderson will complete tests of a new tool to estimate the masses of galaxies outside our Milky Way, based on observations of a particular type of tidal stream known as a shell or umbrella. In these special cases we can understand the stream stars' orbits, and hence the mass of the host galaxy, even with only the limited information we can obtain for such distant stars. The information from these new strategies will allow Dr. Sanderson to compare the structures of galaxy halos, including our Milky Way's, to predictions based on different theories of dark matter. The images, movies, and computer simulations produced in her work will be incorporated into the middle school science lessons she will develop, to help educate and inspire a new generation of American scientists.
