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

Abstract

By means of a series of N-body/SPH simulations we investigate the morphological properties of thick stellar disks formed through minor mergers with, e.g. a range of gas-to-stellar mass ratios. 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 furthest away from the disk mid-plane (z 2 kpc). This stellar excess also follows a sech function with a larger scale height than the main thick disk component (except at large radii). It is usually dominated by stars from the primary galaxy, but this depends on the orbital configuration. 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, which can have a similar lag. Confirming previous results, 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, i.e., the disk heating effect of a few consecutive mergers does not saturate but is cumulative. 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 ("clumpy disks"). These clumpy disks do not produce either a stellar excess or a ratio of thick to thin disk scale lengths greater than one. Comparing our simulation results with observations of the Milky Way and nearby galaxies shows that our results for minor mergers are consistent with observations of the ratio of thick to thin disk scale lengths and with the "Toomre diagram" (the sum in quadrature of the vertical and radial versus the rotational kinetic energies) of the Milky Way. The simulations of clumpy disks do not show such agreement. 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.

Two distinct halo populations in the solar neighborhood

“…All together, this suggests that the low-α stars have been accreted from satellite galaxies including the progenitor galaxy of ω Cen (Bekki & Freeman 2003). The high-α stars, on the other hand, are more likely to be formed "in situ" in the Galactic bulge or disk, and then "heated" to halo kinematics by the merging satellites, as suggested from numerical simulations (Zolotov et al 2009(Zolotov et al , 2010Purcell et al 2010;Qu et al 2011).…”

The age structure of stellar populations in the solar vicinity

“…Heating during the merger epoch (Quinn et al 1993;Qu et al 2011;House et al 2011), or scattering of stars by massive clumps (Bournaud et al 2009) may have caused additional thickening of the already formed thick stellar component (Quinn et al 1993;Qu et al 2011).…”

On the self-regulation of intense star-formation in galaxies atz= 1−3

“…This model has a high gas mass fraction (50%) and the galaxy was evolved in isolation, without companions or tidal interactions (Qu et al 2011). We allowed the disk to go unstable against star-formation and it developed a "clumpy" morphology which evolved with time (see Di Matteo et al 2008, for details of the initial conditions and evolution of the simulation).…”