The Secular Evolution of Disk Structural Parameters

We present a comprehensive series of $N$-body as well as $N$-body + SPHsimulations to study the secular evolution of the structure of disk galaxies.Our simulations are organized in a hierarchy of increasing complexity, rangingfrom rigid-halo collisionless simulations to fully live simulations with gasand star formation. Comparisons between the different types of simulationsallow us to isolate the role of various physical mechanisms. We focus on theevolution of systems expected in a LCDM universe. Our goal is to examine which structural properties of disk galaxies mayresult from secular evolution rather than from direct hierarchical assembly. Inthe vertical direction, we find that various mechanisms can lead to heating.The strongest heating occurs during the vertical buckling instability of a bar.Among the consequences of this instability is the formation of peanut-shapedbulges which produce clear kinematic signatures when observed face-on. We findthat bars are robust structures that are not destroyed by buckling. They can bedestroyed instead by a central mass concentration but we find that this massneeds to be a large fraction of the total mass of the disk. We then study theevolution of stellar surface density profiles showing how angular momentumredistribution leads to increasing central densities and disk scale lengths andto profile breaks at large radii. The breaks in these simulations are inexcellent agreement with observed breaks, even when the evolution is purelycollisionless. Disk scale-lengths increase even when the total disk angularmomentum is conserved; thus mapping halo angular momenta to scale-lengths isnon-trivial. [Abridged]