0 Assigned ID: 2370 ----- Attempting Submission 1 (Fri Apr 09 22:35:06 GMT 2021) ----- HST Phase I Proposal 2370 successfully submitted. Receipt: # 2370-1 ----- Attempting Submission 2 (Fri Apr 09 22:49:27 GMT 2021) ----- HST Phase I Proposal 2370 successfully submitted. Receipt: # 2370-2 ----- Attempting Submission 3 (Fri Apr 09 23:40:29 GMT 2021) ----- The Milky Way's stellar halo contains information about the Galaxy's accretion history, its growth through time, and its dynamical interactions with satellites. From our location in the Galaxy, we can use measurements of individual stars to trace the history of our Galaxy's satellite accretion, measuring inhomogeneity and substructure in the kinematics and abundances in the halo caused by these interactions. To measure these properties, full positional, kinematic, and chemical abundance data is needed for halo stars along many lines of sight. Spectra can constrain distance and measure line of sight velocity and chemistry, but the tangential velocities require measuring proper motions (PMs). Precise astrometry with HST has been used to determine PMs for stars in the Milky Way halo from deep, multi-epoch data, but this is only available for a few fields. Gaia's third data release (EDR3) offers unprecedented spatial coverage by providing PMs across the whole sky, but the relatively short time baseline limits its accuracy for faint stars. Here, we propose to develop tools that will combine single-epoch HST archival images with Gaia measurements to measure more accurate PMs for many more stars at G>18 mag on many more fields in the halo. This will enhance the scientific impact of both observatories beyond their individual capabilities. Archival HST images will be matched with Gaia to increase the PM precision for faint halo stars in Gaia's catalog (18<G<21 mag). The new PMs and existing spectra will be used to measure multi-dimensional information for a large sample of halo stars. We will use this 7D data to obtain new constraints on the accretion history of our Galaxy. K. McKinnon (UCSC PhD student): will head the adaptation of the proper motion measurement pipeline for sparse fields using his background in statistical modelling. He has begun the adaptation process, and investigated the potential for using a time-series analysis for improving the transformations. He previously measured [Fe/H] and [alpha/Fe] for the HALO7D survey (Cunningham et al. 2016, 2019a, 2019b) main sequence stars in the CANDELS fields and will do so for the new lines of sight. For his thesis, he will extend the kinematic modeling of the halo to include the additional dimensions of [Fe/H] and [alpha/Fe]. M. Apfel (UCSC PhD student) will receive the other half of the proposed student support. She will assist on the adaptation of the pipeline. She led the selection of appropriate HST fields for which we plan to measure proper motions using this technique, and investigated the power of our survey in distinguishing between accretion histories. For her thesis, Apfel will lead the mapping of spatial variations in the combined kinematics and chemical abundances and compare with simulations of stellar halo formation in Milky Way-like galaxies. C. Rockosi and P. Guhathakurta (UCSC) will be the co-advisors of the two UCSC PhD students on the project, and will be responsible for the successful completion of the program. Both will lend their expertise on metallicity measurements from medium resolution spectra, accurate stellar velocities, and dynamical analysis. They previously worked on the HALO7D survey, which used HST-based 3D kinematics to investigate inhomogeneities in the distant stellar halo. E. Cunningham (Center for Computational Astrophysics at the Flatiron Institute) lead the original HALO7D survey to investigate the distant stellar halo using HST-based 3D kinematics. She will contribute her expertise to mentoring Apfel and McKinnon on proper motion measurements and modeling of the 7D data. R. van der Marel, A. del Pino, M. Libralato, J. Anderson, S. T. Sohn, and L. Watkins (STScI) developed the original pipeline for combining single-epoch HST and Gaia to measure proper motions in crowded fields. They developed the method to measure accurate PMs in deep multi-epoch HST images using distant galaxies as the astrometric reference frame, the method used for the original HALO7D survey. Stellar Populations and the Interstellar Medium 0 Regular false false false false false SUPPORTED