  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, Delta z_d, depends
strongly on the individual mass of an interacting subhalo M_{sub}. This is
described by the relation, Delta z_d / R_d = 8 Sum_{j=1}^N (M_{sub,j}/M_d)**2,
where R_d is a disk scale length, M_d is a disk mass, and N is the total number
of accretion events of subhalos inside a disk region (< 3R_d). 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 V_c is more affected by subhalos than a
disk with larger V_c, in agreement with the observation. Further implications
of our results for the origin of a thick disk component are also discussed.

Astrophysics