  A fast $N$-body code has been developed for simulating a stellar disk
embedded in a live dark matter halo. In generating its Poisson solver, a
self-consistent field (SCF) code which inherently possesses perfect scalability
is incorporated into a tree code which is parallelized using a library termed
Framework for Developing Particle Simulators (FDPS). Thus, the code developed
here is called SCF-FDPS. This code has realized the speedup of a conventional
tree code by applying an SCF method not only to the calculation of the
self-gravity of the halo but also to that of the gravitational interactions
between the disk and halo particles. Consequently, in the SCF-FDPS code, a tree
algorithm is applied only to calculate the self-gravity of the disk. On a
many-core parallel computer, the SCF-FDPS code has performed at least three
times, in some case nearly an order of magnitude, faster than an
extremely-tuned tree code on it, if the numbers of disk and halo particles are,
respectively, fixed for both codes. In addition, the SCF-FDPS code shows that
the cpu cost scales almost linearly with the total number of particles and
almost inversely with the number of cores. We find that the time evolution of a
disk-halo system simulated with the SCF-FDPS code is, in large measure, similar
to that obtained using the tree code. We suggest how the present code can be
extended to cope with a wide variety of disk-galaxy simulations.
