NOAO >   Observing Info >   Approved Programs >   2014A-0530

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Proposal Information for 2014A-0530


PI: Tuan Do, University of Toronto, do@di.utoronto.ca
Address: Department of Astronomy, 60 St. George Street, Toronto, ON M5S 1A1, Canada

CoI: Jessica Lu, University of Hawaii (IfA)
CoI: Josh Simon, Carnegie Institution of Washington (Carnegie Obs.)
CoI: Annika Peter, Ohio State University
CoI: Mike Boylan-Kolchin, University of Maryland

Title: Measuring the orbital history of the ultra-faint dwarf galaxy Hercules with GSAOI

Abstract: The Milky Way ultra-faint dwarf galaxies are the most dark matter dominated systems known to date. Their low masses, low luminosities, and extremely low metallicities offer a glimpse into galaxy formation at the earliest epochs, while their high inferred dark matter densities and proximity make them ideal candidates for indirect dark matter detection experiments. However, significant uncertainties remain in key observables such as the mass and infall history of these extreme objects. For example, without knowledge of their orbits, it is difficult to determine whether their masses are overestimated because their radial velocity dispersions have been inflated by past tidal encounters with the Milky Way. We propose to measure the proper motion of the Hercules ultra-faint dwarf galaxy using GSAOI to understand its orbital history. Hercules is a particularly intriguing target because structural and kinematic studies have motivated claims that it is tidally disrupting despite its relatively large present distance from the Milky Way and apparently high mass-to-light ratio. It also has a very old stellar population, making it a prime candidate for a "fossil galaxy" whose star formation was shut off by reionization. With observations in 2014A, 2015A, and 2017A in conjunction with HST data taken in 2011, we will be able to achieve 35 km/s proper motion precision, making it possible to reconstruct the orbit of Hercules. Knowledge of its perigalacticon will allow us to quantify the tidal effects of the Milky Way on its internal stellar dynamics, while its eccentricities and orbital energies will constrain the initial infall time into the Milky Way dark matter halo. This proposal aims to extend the technique developed of HST data to using background galaxies as absolute reference sources to ground based MCAO observations, which will be important for astrometry work after HST and for future extremely large telescopes.


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