Abstract

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 (lsim 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 (gsim 0.4) have orientations (Δθ_GC) in the range 20° <~ Δθ_GC <~ 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.

This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile and the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.