NGC 147, NGC 185 and CassII: a genetic approach to orbital properties, star formation and tidal debris

TL;DR

This study uses a genetic algorithm and N-body simulations to explore the orbital history of three M31 satellites, explaining their diverse star formation and tidal features while showing they can be gravitationally bound over a gigayear.

Contribution

It introduces a novel genetic algorithm approach combined with N-body simulations to model the complex orbital interactions of satellite galaxies around M31.

Findings

NGC147, NGC185, and CassII can be gravitationally bound for over a gigayear.

Different orbital interactions explain the contrasting tidal features.

The models reproduce observed stellar streams and lack of tidal features in the satellites.

Abstract

NGC147, NGC185 and CassiopeiaII (CassII) have similar positions in the sky, distances and measured line of sight velocities. This proximity in phase space suggests that these three satellites of M31 form a subgroup within the Local Group. Nevertheless, the differences in their star formation history and interstellar medium, and the recent discovery of a stellar stream in NGC~147, combined with the lack of tidal features in the other two satellites, are all indications of complex and diverse interactions between M31 and these three satellites. We use a genetic algorithm to explore the different orbits that these satellites can have and select six sets of orbits that could best explain the observational features of the NGC147, NGC185 and CassII satellites. The parameters of these orbits are then used as a starting point for N-body simulations. We present models for which NGC147, NGC185 and CassII are a bound group for a total time of at least one Gyr but still undergo different interactions with M31 and as a result NGC147 has a clear stellar stream whereas the other two satellites have no significant tidal features. This result shows that it is possible to find solutions that reproduce the contrasting properties of the satellites and for which NGC147-NGC185-CassII have been gravitationally bound.