Simulations of the heating of the Galactic stellar disc
  The velocity dispersion of nearby stars in the Galactic disc is well known to increase substantially with age; this is the so-called Age-Velocity relation, and is interpreted as a ``heating'' of the disc as a function of time. We have studied the heating of the disc due to giant molecular clouds and halo black holes, using simulations of the orbits of tracer stars embedded in a patch of the local Galactic disc. We examine a range of masses and number densities of the giant molecular cloud and halo black hole perturbers. The heating of the stellar disc in the simulations is fit with a simple power law, and we also fit this form to the best determinations of Age-Velocity relation as derived from stars in the solar neighbourhood for which ages can be reliably assigned. Observationally, the Age-Velocity relation remains poorly constrained and its determination is probably still dominated by systematic errors. Our simulations confirm the well known results that there are insufficient GMCs to heat the Galactic disc appropriately. A range of dark halo black hole scenarios are verified to heat the stellar disc in the manner expected from analytical studies, and they reproduce the observed ratio of the stellar velocity dispersions. Simulations featuring a combination of giant molecular clouds and halo black holes can explain the observed heating of the stellar disc, but since other perturbing mechanisms, such as spiral arms, are yet to be included,we regard this solution as ad hoc. 
Jyrki Hanninen (Tuorla Observatory) and Chris Flynn (Tuorla   Observatory & Swinburne University)