Dr. Zinn and his team study the accretion history of the halo of the Milky Way galaxy. This work tests the accretion paradigm and the dual halo picture. If the halo was built up over billions of years through the accretion and tidal disruption of many dwarf satellite galaxies, it will contain streams of stars from disrupted galaxies that are detectable as regions of enhanced stellar density (halo substructures) and as stars moving together through space. The outer halo may consist primarily of stars from tidally destroyed satellite galaxies, whereas the inner halo may have originated in situ from accreted gas or within the very first objects that merged to form the Galaxy. It is recognized that the halo has two major components that differ in stellar kinematics, spatial distribution, and metallicity distribution. However only a small fraction of the halo has been surveyed, and properties of some known halo substructures are poorly determined. The different sub-structures and streams in the halo are traceable with RR Lyrae stars. These pulsating stars are easily recognized by their half-day periods and about one magnitude amplitudes. This research will provide better distances and sizes of previously detected halo substructures and look for new substructures in yet to be surveyed areas of the sky. This work uses RR Lyrae stars found in the LaSilla-QUEST survey up to about 60 kpc from the Sun. Spectroscopic observations are used to identify if suspected substructures are real groups of co-moving stars, as expected for remains of accreted satellite galaxies. Radial velocities and metallicities are measured for about 150 RR Lyrae stars in the inner halo to test whether RR Lyrae stars separate into two components, or are instead concentrated in the accreted halo. This project trains graduate and undergraduate students in the techniques of optical photometry and spectroscopy. As part of the outreach activities, a show on the evolution of the Milky Way galaxy will be created for Yale's Leitner Family Observatory and Planetarium.
