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