  The formation of thick stellar disks in spiral galaxies is studied.
Simulations of gas-rich young galaxies show formation of internal clumps by
gravitational instabilities, clump coalescence into a bulge, and disk
thickening by strong stellar scattering. The bulge and thick disks of modern
galaxies may form this way. Simulations of minor mergers make thick disks too,
but there is an important difference. Thick disks made by internal processes
have a constant scale height with galactocentric radius, but thick disks made
by mergers flare. The difference arises because in the first case,
perpendicular forcing and disk-gravity resistance are both proportional to the
disk column density, so the resulting scale height is independent of this
density. In the case of mergers, perpendicular forcing is independent of the
column density and the low density regions get thicker; the resulting flaring
is inconsistent with observations. Late-stage gas accretion and thin disk
growth are shown to preserve the constant scale heights of thick disks formed
by internal evolution. These results reinforce the idea that disk galaxies
accrete most of their mass smoothly and acquire their structure by internal
processes, in particular through turbulent and clumpy phases at high redshift.
