Speaker
Description
Determination of the mass density profiles of dwarf galaxies (and specifically whether there is a central core or cusp) provides a critical test of both the properties of dark matter (DM) and the physics of cosmological structure formation. The nearby classical dwarf spheroidal galaxies (dSphs) of the Milky Way yield some of the best dynamical constraints. While large line-of-sight velocity datasets exist (some thousand stars per galaxy), interpretation is hindered by the well-known mass vs. velocity-anisotropy degeneracy of stellar dynamics. This can be resolved with proper motion (PM) measurements that yield 3-D velocity information, which is beyond the reach of Gaia, given the small velocity dispersions of dSphs and the absence of bright stars. To attain the necessary precision and a proper handling of systematics for this kind of study, one then needs not only longer baselines, but the combination of many fields from different state-of-the-art telescopes. We thus obtain separate PMs from the nearby Draco and Sculptor dSphs from 3 epochs of HST data for 5 fields, from 2 epochs of new JWST observations of these same fields as well as two additional fields, all of which are further compared to Gaia DR3 positions. From this long-term program we will be able to provide a direct determination of their velocity anisotropy profiles, and combined with dynamical models, tightly constrain the slopes of their DM density profiles. No comparable measurements exist to date, and the precision attainted will be fundamental to lay more robust constraints on both the nature of DM, and the physical mechanisms that shape DM density profiles in galaxies. We will discuss the progress of our program, and some initial results from the observational epochs and data obtained to date. Additional observations to be obtained through 2025 will yield the final accuracies needed for our goals.
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