Birds of a Feather? Magellan/IMACS Spectroscopy of the Ultra-faint Satellites Grus II, Tucana IV, and Tucana V
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AbstractWe present Magellan/IMACS spectroscopy of three recently discovered ultra-faint Milky Way satellites, GrusII, TucanaIV, and TucanaV. We measure systemic velocities of m{s}}}^{-1}$?> , m{s}}}^{-1}$?> , and m{s}}}^{-1}$?> for the three objects, respectively. Their large relative velocities demonstrate that the satellites are unrelated despite their close physical proximity. We determine a velocity dispersion for TucIV of m{s}}}^{-1}$?> , but we cannot resolve the velocity dispersions of the other two systems. For GruII, we place an upper limit (90%confidence) on the dispersion of <1.9 m{s}}}^{-1}$?> , and for TucV, we do not obtain any useful limits. All three satellites have metallicities below m{H}}]=-2.1$?> , but none has a detectable metallicity spread. We determine proper motions for each satellite based on Gaia astrometry and compute their orbits around the Milky Way. GruII is on a tightly bound orbit with a pericenter of kpc and orbital eccentricity of . TucV likely has an apocenter beyond 100kpc and could be approaching the Milky Way for the first time. The current orbit of TucIV is similar to that of GruII, with a pericenter of kpc and an eccentricity of . However, a backward integration of the position of TucIV demonstrates that it collided with the Large Magellanic Cloud at an impact parameter of 4kpc 120Myr ago, deflecting its trajectory and possibly altering its internal kinematics. Based on their sizes, masses, and metallicities, we classify GruII and TucIV as likely dwarf galaxies, but the nature of TucV remains uncertain.
