Dynamical Effects of CDM Subhalos on a Galactic Disk

We investigate the dynamical interaction between a galactic disk and surrounding numerous dark subhalos as expected for a galaxy-sized halo in the cold dark matter (CDM) models. Our particular interest is to what extent accretion events of subhalos into a disk are allowed in light of the observed thinness of a disk. Several models of subhalos are considered in terms of their internal density distribution, mass function, and spatial and velocity distributions. Based on a series of N-body simulations, we find that the disk thickening quantified by the change of its scale height, ∆zd, depends strongly on the individual mass of an interacting subhalo Msub. This is described by the relation, ∆zd/Rd ≃ 8 ∑N j=1(Msub,j/Md) , where Rd is a disk scale length, Md is a disk mass, and N is the total number of accretion events of subhalos inside a disk region (≤ 3Rd). Using this relation, we find that an observed thin disk has not ever interacted with subhalos with the total mass of more than 15 % disk mass. Also, a less massive disk with smaller circular velocity Vc is more affected by subhalos than a disk with larger Vc, in agreement with the observation. Further implications of our results for the origin of a thick disk component are also discussed.