A New Proper Motion Determination of Leo I

TL;DR

This paper presents a highly precise measurement of Leo I's absolute proper motion using HST data spanning 10 years, aligning its orbital pole with the Vast Polar Structure of Milky Way satellites.

Contribution

It provides the longest baseline proper motion measurement of Leo I and improves astrometric calibration, offering a new, accurate proper motion value consistent with Gaia data.

Findings

Proper motion of Leo I is (-0.036, -0.130) mas/yr.

Leo I's orbital pole aligns with the Vast Polar Structure.

Measurement confirms previous Gaia-based proper motion estimates.

Abstract

We measure the absolute proper motion of Leo I using a WFPC2/HST data set that spans up to 10 years, to date the longest time baseline utilized for this satellite. The measurement relies on ~ 2300 Leo I stars located near the center of light of the galaxy; the correction to absolute proper motion is based on 174 Gaia EDR3 stars and 10 galaxies. Having generated highly-precise, relative proper motions for all Gaia EDR3 stars in our WFPC2 field of study, our correction to the absolute EDR3 system does not rely on these Gaia stars being Leo I members. This new determination also benefits from a recently improved astrometric calibration of WFPC2. The resulting proper-motion value, (mu_alpha, mu_delta) = (-0.007 +- 0.035, -0.119 +-0.026) mas/yr is in agreement with recent, large-area, Gaia EDR3-based determinations. We discuss all the recent measurements of Leo I's proper motion and adopt a combined, multi-study average of (mu_alpha_3meas, mu_delta_3meas) = (-0.036 +- 0.016, -0.130 +- 0.010) mas/yr. This value of absolute proper motion for Leo I indicates its orbital pole is well aligned with that of the Vast Polar Structure, defined by the majority of the brightest dwarf-spheroidal satellites of the Milky Way.