Heating of Galactic Disks by Infalling Satellites

  • 15 July 2003
Abstract
We develop an analytic model to calculate the rate at which galaxy disks are heated by dark matter substructures orbiting in their halos. We calibrate this model against N-body simulations and demonstrate that it reproduces the N-body heating rates to within a factor of 2 in the majority of cases. Applying these calculations within a model for the growth of galaxies in a Lambda-dominated CDM universe, we predict the distribution of disk scale heights for galaxies of different luminosities. For L* spiral galaxies, we predict that the median disk thickness is only 5% of the radial scalelength if substructure is the only source of disk heating. The median disk thickness increases to nearly 20% of the radial scalelength when heating due to gravitational scattering of stars by molecular clouds is included. This latter value is close to the thickness estimated observationally for the Milky Way. The distribution of thicknesses predicted by the model is consistent with a recent observational determination for sub-L* disk galaxies by Bizyaev & Mitronova. Thus, the observed thicknesses of the disks of spiral galaxies seem to be entirely consistent with the abundance of substructure in dark matter halos predicted by the standard Lambda-dominated CDM model. Universes with Omega_0=1 predict similar scale-heights for our best model of galaxy formation in this cosmology, a consequence of the fact that similar amounts of substructure are accreted by halos in Omega_0=1 and Omega_0=0.3, Lambda_0=0.7 cosmologies during the lifetimes of their respective disks.

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