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

TL;DR

This study uses high-resolution galaxy simulations to investigate the formation of disk outskirts, revealing a persistent surface density break caused by star formation cutoff and stellar redistribution, aligning well with observations.

Contribution

It demonstrates how a combination of star formation thresholds and secular stellar redistribution explains the observed disk break features in galaxy outskirts.

Findings

The surface density profile shows an inner exponential with a steeper outer exponential.

The break forms early and moves outward as the galaxy evolves.

Stars beyond the break are scattered from the inner disk by spiral arms.

Abstract

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.