Beyond Inside-Out Growth: Formation and Evolution of Disk Outskirts

Abstract

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