Dr. Weinberg and his team develop and apply modeling methods to interpret measurements of radial velocities and multi-element chemical abundances from large spectroscopic surveys of tens or hundreds of thousands of stars. Such studies are necessary to improve our understanding of the physical processes in galaxy formation and galactic chemical evolution. The primary focus for near-term applications is to use data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey, which is a follow-up survey of the Sloan Digital Sky Survey (SDSS-III). This survey will measure precise radial velocities and multi-element chemistry (about 15 elements per star) for about 100,000 red giant stars in all regions of the Milky Way. Other target data sets include the Sloan Extension for Galactic Understanding and Exploration survey (SEGUE, another follow up survey to SDSS), and the Radial Velocity Experiment (RAVE) survey of the solar neighborhood, but also data from other large surveys planned in the future .The research employs three complementary modeling approaches: Analytic chemical evolution models that incorporate radial mixing and dynamical processes, N-body/hydrodynamic simulations that model growth of a galaxy disk either in isolation or in the presence of accreting satellite galaxies, and simulations of galactic disk formation from fully cosmological initial conditions. The growing disk simulations use the Gasoline code, which is also used in the cosmological runs, but gas is added to the disk as a prescribed function of radius and time. This approach provides controlled simulations that can isolate the effects of specific physical processes, and realize distinct scenarios that can be used to distinguish processes such as galactic bulge formation by secular evolution versus formation through minor mergers. The investigators will develop and apply the technique of Principal Component Abundance Analysis (PCAA) to characterize any correlated variations of elemental abundances in large stellar data sets. The PCAA tool will be a valuable tool for understanding systematic effects and statistical errors in the APOGEE chemical abundance data and for characterizing its multi-dimensional measurements in compact form. This and other tools are likely to increase the impact of the SDSS-III data sets. For example, by applying these tools to the interpretation of APOGEE and other data sets, the investigators can address questions about the origin of the Galactic thick disk, bulge, and stellar halo, and the history of chemical enrichment and migration of stars within the thin disk. The PCAA and data-model comparisons are used to analyze the distribution of stellar chemical compositions as a function of metallicity and spatial location, and to characterize the level of chemo-dynamical substructure in different components of the Galaxy. This can constrain the importance of satellite perturbations and mergers, and by identifying the principal patterns of enrichment and rare outliers from these patterns, one can gain insights insight into the key pathways of stellar nucleosynthesis. Graduate students and postdoctoral researchers are involved in this project and will be trained in developing and applying theory, observation, and statistical analysis tools to problems in Galaxy formation and evolution.
