Studying the Accretion History of the Milky Way Using Mock Observations of Galaxy Formation Simulations

In the Lambda Cold Dark Matter hierarchical structure formation paradigm, a spiral galaxy’s stellar halo is thought to have formed through the accretion of smaller dwarf galaxies onto the larger galaxy. The remnant stars from each accretion event trace out a different shape or morphology in the stellar halo, and as a whole these features are the substructure of the stellar halo. The amount and morphologies of substructural features helps determine the accretion history of the Milky Way, information which helps constrain models of the cosmological evolution of galaxies. The HALO7D survey uses 7D observations (3D positions, 3D velocities, and metallicity) of halo stars to determine the Milky Way’s accretion history; the purpose of our research is to design future HALO7D observations by choosing optimal sky locations for Keck observations and modelling observations of simulated galaxy halos to estimate how much information can be obtained from Keck observations. Based on the overlap of existing Hubble Space Telescope archival images with appropriate depth, availability of reliable Gaia astrometric data, sky coordinate ranges that ensure accessibility from the Keck telescope at different times of the year, low interstellar dust extinction, and the lack of bright/extended galaxies or nebulae, we have identified 12 lines of sight (LOS) for follow up spectroscopic observations. Utilizing Bullock and Johnston (2005) stellar halo simulations, we find that the distribution of observed stellar radial velocities is dependent on the time of accretion, with earlier accretion resulting in more homogeneous distributions across different LOS and late accretion resulting in greater differences across different LOS. These results show that the HALO7D survey promises to constrain broad accretion trends in the Milky Way’s history. Further analysis is required to determine the dependence of these constraints on the specific subsample of stars observed, the location of the Sun within the Milky Way, the number of stars per LOS, and the number of LOS. Comparing the distributions of 3D positions, 3D velocities, and chemical abundances across the different LOS will give further insight on the robustness of the conclusions one can draw from the HALO7D survey.

WH, AP, JS, and PG conducted their research under the auspices of the Science Internship Program (SIP) at the University of California Santa Cruz; PG was a fellow of the Cal Poly San Luis Obispo STEM Teacher and Researcher (STAR) program. This research was funded in part by the National Science Foundation and NASA/STScI.