  In order to explain disk galaxy formation within the hierarchical structure
formation, it seems that gas rich mergers must play an important role. We
review here our previous studies which have shown the importance of mergers at
high redshift being gas rich, in the formation of both the stellar halo and
thick disk components of disk galaxies. Regulation of star formation in the
building blocks of our galaxy is required to form a low mass low metallicity
stellar halo. This regulation results in high redshift, gas rich mergers during
which the thick disk forms. In these proceedings, we categorise stars from our
simulated disk galaxy into thin and thick disk components by using the Toomre
diagram.
