The role of resonances in the vertical heating of disks

Abstract

Vertical heating is a process by which the vertical component of the velocity dispersion increases, leading to structure formation, and is an example of secular evolution. Many mechanisms have been proposed as the causes behind this process. In this investigation, we employ the use of an N-body + SPH simulation of an isolated spiral-barred galaxy model from which the actions can be computed for an axisymmetric potential. We report a cooling effect proportional to the increase in height above the midplane for the change in each respective action, which is compatible with effects arising from spiral resonances. A heating process is also observed to occur below 3 kpc, for the change in radial action and angular momentum respectively, which may be attributed to the presence of the bar. In this respect, we report a good agreement with previous results regarding the extent of variation in the fractional change of the vertical action. For stars within the solar neighbourhood, upon colouring the angular momentum space in ⟨∆Jz⟩, we report the presence of regions of vertical heating. Such regions of vertical heating were found to correspond with stars exhibiting minimal radial migration, in agreement with previous literature. Furthermore, for stars on near-circular orbits, the regions were observed to become more prominent, implying that stars on highly circular orbits are more susceptible to vertical heating due to their highly specific orbital frequencies. Upon investigating the location of the CR and OLR in this action space, we report that the 5:1 vertical OLR and the 3:1 epicyclic OLR were consistently found in close proximity to these heated regions throughout the evolution of the galaxy. If confirmed, this result may imply that the odd multiplicities of the natural frequencies may play a considerable role in the formation of regions of vertical heating within the galaxy.