Boötes III is a Disrupting Dwarf Galaxy Associated with the Styx Stellar Stream

We present proper motion (PM) measurements of Boötes III, an enigmatic stellar satellite of the Milky Way (MW), utilizing data from the second data release of the Gaia mission. By selecting 15 radial velocity confirmed members of Boötes III, along with a likely RR Lyrae member in the vicinity, we measure an error-weighted mean PM of (μ μ _δ) = (-1.14, -0.98)+(0.18, 0.20) mas yr^-1. We select and present further stars that may be Boötes III members based on their combined PM and position in the color-magnitude diagram. We caution against assigning membership to stars that are not confirmed spectroscopically, as we demonstrate that there are contaminating stars from the disrupting globular cluster NGC 5466 in the vicinity of the main body of Boötes III, but we note that our results are consistent with previous Boötes III PM estimates that did not include spectroscopic members. Based on the measured PM and other known properties of Boötes III, we derive its Galactocentric velocity and compute its orbit given canonical MW potentials with halo masses of both 0.8 10¹² M _o and 1.6x10¹² M _o. These orbits robustly show that Boötes III passed within ∼12 kpc of the Galactic center on an eccentric orbit roughly ∼140 Myr ago. Additionally, the PM of Boötes III is in excellent agreement with predictions for the retrograde motion of the coincident Styx stellar stream. Given this, along with the small pericenter and metallicity spread of Boötes III itself, we suggest that it is a disrupting dwarf galaxy giving rise to the Styx stellar stream.