Simulations of minor mergers - I. General properties of thick discs

Abstract

We present simulations of the formation of thick discs via the accretion of two-component satellites onto a pre-existing thin disc. Our goal is to establish the detailed characteristics of the thick discs obtained in this way, as well as their dependence on the initial orbital and internal properties of the accreted objects. We find that mergers with 10–20 per cent mass of the host lead to the formation of thick discs whose characteristics are similar, both in morphology as in kinematics, to those observed. Despite the relatively large mass ratios, the host discs are not fully destroyed by the infalling satellites: a remaining kinematically cold and thin component containing ∼15–25 per cent of the mass can be identified, which is embedded in a hotter and thicker disc. This may for example, explain the existence of a very old thin disc stars in the Milky Way. The final scaleheights of the discs depend both on the initial inclination and properties of the merger, but the fraction of satellite stellar particles at ∼4 scaleheights directly measures the mass ratio between the satellite and host galaxy. Our thick discs typically show boxy isophotes at very low surface brightness levels (>6 mag below their peak value). Kinematically, the velocity ellipsoids of the simulated thick discs are similar to that of the Galactic thick disc at the solar radius. The trend of with radius is found to be a very good discriminant of the initial inclination of the accreted satellite. In the Milky Way, the possible existence of a vertical gradient in the rotational velocity of the thick disc as well as the observed value of at the solar vicinity appear to favour the formation of the thick disc by a merger with either low or intermediate orbital inclination.