Effects of resolution and local stability on galactic disks: Halo resolution and softening on bar formation

¹ Leibniz-Institüt für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Universität Potsdam, Institut für Physik und Astronomie, Karl-Liebknecht-Str. 24–25, 14476 Potsdam, Germany
³ Department of Astronomy and Yonsei University Observatory, Yonsei University, Seoul 03722, Republic of Korea

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 March 2026
Accepted: 16 June 2026

Abstract

Using N-body simulations, we examined the impact of dark matter (DM) halo resolution and gravitational softening on bar formation. We generated isolated disk-halo systems with fixed stellar disk parameters, varying the number of halo particles, softening lengths, and halo concentration to modulate disk stability via the central DM fraction. The effects of DM resolution (m_DM/m_* = 1, 10, and 100) on bar formation are less pronounced in more unstable disks, in which the overall evolutionary path is similar except that the lowest DM resolution model suffers gradual bar weakening. Irrespective of the halo resolution, large softening, ε_DM, flattens the central halo density profile within the softening scale, impeding angular momentum transfer to the nascent bar and preventing bar formation in more stable models. In unstable models with ε_DM = 0.96 kpc, a small bar still emerges due to enhanced initial instability and a larger seed perturbation, yet the bar strength F₂ remains capped around 0.3 owing to unresolved central dynamical friction. Despite the destabilizing effect of the reduced central DM fractions, our results indicate that deficient central angular momentum exchange can still suppress bar growth. Furthermore, halo softening influences buckling instability, as larger values (ε_DM = 0.30 and 0.60 kpc) inhibit central vertical heating, exacerbating radial-vertical velocity dispersion anisotropies and triggering stronger buckling. We recommend adopting m_DM/m_* ≤ 10, m_* ≤ 10⁴ M_⊙, and ε_DM < 0.30 kpc when investigating the formation and evolution of non-axisymmetric structures in Milky Way-mass galaxies.