Proper Motion of the Leo II Dwarf Galaxy Based On Hubble Space Telescope Imaging

Abstract

This article reports a measurement of the proper motion of Leo II, a dwarf galaxy that is a likely satellite of the Milky Way, based on imaging with the Hubble Space Telescope and Wide Field Camera 3. The measurement uses compact background galaxies as standards of rest in both channels of the camera for two distinct pointings of the telescope and a QSO in one channel for each pointing, resulting in the weighted average of six measurements. The measured proper motion in the the equatorial coordinate system is (mu_alpha, mu_delta) = (-6.9 +- 3.7, -8.7 +- 3.9) mas/century and in the Galactic coordinate system is (mu_l, mu_b) = (6.2 +- 3.9, -9.2 +- 3.7) mas/century. The implied space velocity with respect to the Galactic center is (Pi, Theta, Z) = (-37 +- 38, 117 +- 43, 40 +- 16) km/s or, expressed in Galactocentric radial and tangential components, (V_r, V_tan) = (21.9 +- 1.5, 127 +- 42) km/s. The space velocity implies that the instantaneous orbital inclination is 68 degrees, with a 95% confidence interval of (66 deg, 80 deg). The measured motion supports the hypothesis that Leo II, Leo IV, Leo V, Crater 2, and the globular cluster Crater fell into the Milky Way as a group.

19 pages, 19 figures, 4 tables; submitted to the Astronomical Journal

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• $S^5$: Probing the Milky Way and Magellanic Clouds potentials with the 6-D map of the Orphan-Chenab stream
• Gaia EDR3 proper motions of Milky Way dwarfs I: 3D Motions and Orbits
• Planes of satellites around Milky Way/M31-mass galaxies in the FIRE simulations and comparisons with the Local Group
• Broken into Pieces: ATLAS and Aliqa Uma as One Single Stream
• Evolution of galactic planes of satellites in the EAGLE simulation
• A multi-epoch kinematic study of the remote dwarf spheroidal galaxy Leo II
• The tangential velocity excess of the Milky Way satellites
• Satellite group infall into the Milky Way: exploring the Crater-Leo case with new HST proper motions
• Galactic Orbits of Selected Companions of the Milky Way
• Local Group Proper Motion Dynamics