  We have performed a high mass and force resolution simulation of an idealized
galaxy forming from dissipational collapse of gas embedded in a spherical dark
matter halo. The simulation includes star formation and effects of stellar
feedback. In our simulation a stellar disk forms with a surface density profile
consisting of an inner exponential breaking to a steeper outer exponential. The
break forms early on and persists throughout the evolution, moving outward as
more gas is able to cool and add mass to the disk. The parameters of the break
are in excellent agreement with observations. The break corresponds with a
rapid drop in the star formation rate associated with a drop in the cooled gas
surface density, but the outer exponential is populated by stars that were
scattered outward on nearly circular orbits from the inner disk by spiral arms.
The resulting profile and its associated break are therefore a consequence of
the interplay between a radial star formation cutoff and redistribution of
stellar mass by secular processes. A consequence of such evolution is a sharp
change in the radial mean stellar age profile at the break radius.
