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

TL;DR

This paper measures the proper motion of the Leo II dwarf galaxy using Hubble Space Telescope imaging, providing insights into its velocity and orbital inclination, and supporting the hypothesis of group infall into the Milky Way.

Contribution

The study presents the first precise proper motion measurement of Leo II using HST data, revealing its velocity and orbital parameters, and suggesting group infall with other satellites.

Findings

Proper motion of Leo II measured as (-6.9, -8.7) mas/century in equatorial coordinates.

Space velocity of Leo II is approximately (22, 127) km/s in Galactocentric components.

Orbital inclination of Leo II is about 68 degrees, supporting group infall hypothesis.

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.