Dynamical effects of cold dark matter subhalos on a galactic disk

We have investigated the dynamical interaction between a galactic disk and numerous surrounding dark subhalos as expected for a galaxy-sized halo in the cold dark matter 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 were 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 disk thickening, quantified by the change of its scale height, Δz_d, depends strongly on the individual mass of an interacting subhalo, M_sub. This is described by the relation Δz_d/R_d ≃ 8 Σ_j=1^N(M_sub,j/M_d)², where R_d is the disk scale length, M_d is the disk mass, and N is the total number of accretion events of subhalos inside a disk region (≤ 3 R_d). Using this relation, we find that an observed thin disk has never interacted with subhalos with a total mass of more than 15% of the disk mass. Also, a less-massive disk with a smaller circular velocity, V _c is more affected by subhalos than a disk with a larger V _c, in agreement with observations. Further implications of our results for the origin of a thick disk component are also discussed.