  Observations suggest that the structural parameters of disk galaxies have not
changed greatly since redshift 1. We examine whether these observations are
consistent with a cosmology in which structures form hierarchically. We use
SPH/N-body galaxy-scale simulations to simulate the formation and evolution of
Milky-Way-like disk galaxies by fragmentation, followed by hierarchical
merging. The simulated galaxies have a thick disk, that forms in a period of
chaotic merging at high redshift, during which a large amount of alpha-elements
are produced, and a thin disk, that forms later and has a higher metallicity.
Our simulated disks settle down quickly and do not evolve much since redshift
z~1, mostly because no major mergers take place between z=1 and z=0. During
this period, the disk radius increases (inside-out growth) while its thickness
remains constant. These results are consistent with observations of disk
galaxies at low and high redshift.
