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We use 14-year baseline images obtained with the Wide Field Planetary Camera2 on board the Hubble Space telescope to derive a proper motion for one of theMilky Way's most distant dwarf spheroidal companions, Leo II, relative to anextragalactic background reference frame. Astrometric measurements areperformed in the effective point spread function (ePSF) formalism using our owndeveloped code. An astrometric reference grid is defined using 3,224 stars thatare members of Leo II that are brighter than magnitude 25 in the F814W band. Weidentify 17 compact extra-galactic sources, for which we measure a systemicproper motion relative to this stellar reference grid. We derive a propermotion [\mu_{\alpha},\mu_{\delta}]=[+104+/-113,-33+/-151] microarcseconds/yrfor Leo II in the heliocentric reference frame. Though marginally detected, theproper motion yields constraints on the orbit of Leo II. Given a distance of230 Kpc and a heliocentric radial velocity +79 km/s, and after subtraction ofthe solar motion, our measurement indicates a total orbital motion266.1+/-128.7 km/s in the Galactocentric reference frame, with a radialcomponent +21.5+/-4.3 km/s and tangential component 265.2+/-129.4 km/s. Thesmall radial component indicates that Leo II either has a low-eccentricityorbit, or is currently close to perigalacticon or apogalacticon distance. Wesee evidence for systematic errors in the astrometry of the extragalacticsources which, while close to being point sources, are slightly resolved in theHST images. We argue that more extensive observations at later epochs will benecessary to better constrain the proper motion of Leo II. We provide adetailed catalog of the stellar and extragalactic sources identified in the HSTdata which should provide a solid early-epoch reference for future astrometricmeasurements.

A first measurement of the Proper Motion of the Leo II dwarf spheroidal galaxy