# SCF-FDPS: A Fast $N$-body Code for Simulating Disk-halo Systems

## TL;DR

The paper introduces SCF-FDPS, a fast and scalable N-body simulation code for disk-halo systems that significantly outperforms traditional tree codes in speed while maintaining accurate system evolution.

## Contribution

The paper presents a novel hybrid N-body code combining SCF and tree methods, achieving at least threefold speedup and near-linear scaling on many-core computers.

## Key findings

- SCF-FDPS is at least three times faster than conventional tree codes.
- The code's CPU cost scales almost linearly with total particles.
- The simulated disk-halo evolution closely matches results from traditional tree codes.

## Abstract

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.