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TIDAL SIGNATURES IN THE FAINTEST MILKY WAY SATELLITES: THE DETAILED PROPERTIES OF LEO V, PISCES II, AND CANES VENATICI II

We present deep wide-field photometry of three recently discovered faint Milky Way (MW) satellites: Leo V, Pisces II, and Canes Venatici II. Our main goals are to study the structure and star formation history of these dwarfs; we also search for signs of tidal disturbance. The three satellites have similar half-light radii (∼60–90 pc) but a wide range of ellipticities. Both Leo V and CVn II show hints of stream-like overdensities at large radii. An analysis of the satellite color–magnitude diagrams shows that all three objects are old (>10 Gyr) and metal-poor ([Fe/H] ∼ −2), though neither the models nor the data have sufficient precision to assess when the satellites formed with respect to cosmic reionization. The lack of an observed younger stellar population (≲ 10 Gyr) possibly sets them apart from the other satellites at Galactocentric distances ≳ 150 kpc. We present a new compilation of structural data for all MW satellite galaxies and use it to compare the properties of classical dwarfs to the ultra-faints. The ellipticity distribution of the two groups is consistent at the ∼2σ level. However, the faintest satellites tend to be more aligned toward the Galactic Center, and those satellites with the highest ellipticity (≳ 0.4) have orientations (ΔθGC) in the range 20° ≲ ΔθGC ≲ 40°. This latter observation is in rough agreement with predictions from simulations of dwarf galaxies that have lost a significant fraction of their dark matter halos and are being tidally stripped.