Formation of Andromeda II via a gas-rich major merger and an interaction with M31

TL;DR

This paper models Andromeda II's formation through a gas-rich major merger and a subsequent interaction with M31, using detailed simulations to explain its stellar populations, morphology, and kinematics.

Contribution

It presents a comprehensive simulation-based model combining a high-redshift merger and a later interaction with M31 to explain And II's unique properties.

Findings

Successfully reproduces And II's star formation history

Explains the galaxy's morphology and kinematic features

Accounts for the absence of gas in And II

Abstract

Andromeda II (And II) has been known for a few decades but only recently observations have unveiled new properties of this dwarf spheroidal galaxy. The presence of two stellar populations, the bimodal star formation history (SFH) and an unusual rotation velocity of And II put strong constrains on its formation and evolution. Following Lokas et al. (2014), we propose a detailed model to explain the main properties of And II involving (1) a gas-rich major merger between two dwarf galaxies at high redshift in the field and (2) a close interaction with M31 about 5 Gyr ago. The model is based on N-body/hydrodynamical simulations including gas dynamics, star formation and feedback. One simulation is designed to reproduce the gas-rich major merger explaining the origin of stellar populations and the SFH. Other simulations are used to study the effects of tidal forces and the ram pressure stripping during the interaction between And II and M31. The model successfully reproduces the SFH of And II including the properties of stellar populations, its morphology, kinematics and the lack of gas. Further improvements to the model are possible via joint modelling of all processes and better treatment of baryonic physics.