HST Proper Motions of NGC 147 and NGC 185: Orbital Histories and Test of Dynamically Coherent Andromeda Satellite Plane

TL;DR

This study measures the proper motions of NGC 147 and NGC 185, revealing their orbital histories around M31, supporting the idea that the Andromeda satellite plane is dynamically coherent, and clarifying their binary status.

Contribution

First proper motion measurements for NGC 147 and NGC 185, linking their orbits to the Andromeda plane and testing their binary nature.

Findings

NGC 147's motion aligns with its tidal tails.

NGC 147 and NGC 185 had close passages to M31 within the last 0.5 and 1.6 Gyr.

Orbital poles are consistent with the Great Plane of Andromeda.

Abstract

We present the first proper motion (PM) measurements for the dwarf elliptical galaxies NGC 147 and NGC 185, two satellite galaxies of M31, using multi-epoch HST imaging data with time baselines of $\sim 8$ years. For each galaxy, we take an error-weighted average of measurements from HST ACS/WFC and WFC3/UVIS to determine the PMs. Our final results for the PMs are $(\mu_\mathrm{W}, \mu_\mathrm{N})_\mathrm{N147} = (-0.0232, 0.0378) \pm (0.0143, 0.0146)\,\mathrm{mas}\,\mathrm{yr}^{-1}$ for NGC 147, and $(\mu_\mathrm{W}, \mu_\mathrm{N})_\mathrm{N185} = (-0.0242, 0.0058) \pm (0.0141, 0.0147)\,\mathrm{mas}\,\mathrm{yr}^{-1}$ for NGC 185. The 2-dimensional direction of motion for NGC 147 about M31 is found to be aligned with its tidal tails. The 3-d positions and velocities of both galaxies are transformed into a common M31-centric coordinate system to study the detailed orbital histories of the combined M31+NGC 147+NGC 185 system via numerical orbit integration. We find that NGC 147 (NGC 185) had its closest passage to M31 0.3-0.5~Gyr ($\gtrsim 1.6$~Gyr) within the past 6 Gyr at distances of $\sim 70$ kpc (70-260 kpc). The pericentric times of NGC 147/NGC 185 correlate qualitatively well with the presence/absence of tidal tails seen around the galaxies. Our PMs show that the orbital poles of NGC 147, and also NGC 185 albeit to a lesser degree, agree within the uncertainties with the normal of the Great Plane of Andromeda (GPoA). These are the first measurements of the 3-d angular momentum vector of any satellite identified as original GPoA members. Our results strengthen the hypothesis that the GPoA may be a dynamically coherent entity. We revisit previous claims that NGC 147 and NGC 185 are binary galaxies and conclude that it is very unlikely the two galaxies were ever gravitationally bound to each other.