Characteristics of thick disks formed through minor mergers: stellar excesses and scale lengths

Abstract

By means of N-body/SPH simulations we investigate the morphological properties of thick disks formed through minor mergers. We show that the vertical surface density profile of the post-merger thick disk follows a sech function and has an excess in the regions far from the disk mid-plane (z>2kpc). This stellar excess also follows a sech function with a larger scale height than the main thick disk component, and it is usually dominated by stars from the primary galaxy. Stars in the excess have a rotational velocity lower than that of stars in the thick disk, and they may thus be confused with stars in the inner galactic halo. The thick disk scale height increases with radius and the rate of its increase is smaller for more gas rich primary galaxies. On the contrary, the scale height of the stellar excess is independent of both radius and gas fraction. We also find that the post-merger thick disk has a radial scale length which is 10-50% larger than that of the thin disk. Two consecutive mergers have basically the same effect on heating the stellar disk as a single merger of the same total mass. To investigate how thick disks produced through secular processes may differ from those produced by minor mergers, we also simulated gravitationally unstable gas-rich disks. These disks do not produce either a stellar excess or a ratio of thick to thin disk scale lengths greater than one. Our simulation results are consistent with observations of the ratio of thick to thin disk scale lengths of the Milky Way and nearby galaxies, and with the Toomre diagram of the Milky Way. We conclude that minor mergers are a viable mechanism for the creation of galactic thick disks and investigating stars at several kpc above the mid-plane of the Milky Way and other galaxies may provide a quantitative method for studying the minor merger history of galaxies.