HST Proper Motion of Andromeda III: Another Satellite Co-orbiting The M31 Satellite Plane

TL;DR

This study measures the proper motion of Andromeda III over 22 years, analyzing its orbit and relation to M31's satellite plane, providing insights into satellite dynamics and galaxy mass estimates.

Contribution

It presents a new, precise measurement of Andromeda III's proper motion using advanced data processing and calibration techniques, and analyzes its orbital dynamics within the M31 system.

Findings

Andromeda III's orbit aligns with the M31 satellite plane.

Andromeda III is bound to M31 if M31's mass exceeds 1.5×10^{12} solar masses.

Proper motion measurement achieved with a 22-year baseline.

Abstract

We measure the absolute proper motion of Andromeda III using ACS/WFC and WFPC2 exposures spanning an unprecedented 22-year time baseline. The WFPC2 exposures have been processed using a deep-learning centering procedure recently developed as well as an improved astrometric calibration of the camera. The absolute proper motion zero point is given by 98 galaxies and 16 Gaia EDR3 stars. The resulting proper motion is $(\mu_{\alpha} , \mu_{\delta}) = (-10.5\pm12.5, 47.5\pm12.5)~\mu$as yr$^{-1}$. We perform an orbit analysis of And III using two estimates of M31's mass and proper motion. We find that And III's orbit is consistent with dynamical membership to the Great Plane of Andromeda system of satellites although with some looser alignment compared to the previous two satellites NGC 147 and NGC 185. And III is bound to M31 if M31's mass is $M_{\mathrm{vir}}\geq 1.5\times10^{12}\,M_{\odot}$.