Dependency of the Bar Formation Timescale on the Halo Spin

Abstract

Bars are among the most prominent structures in disk galaxies. While the widely accepted swing-amplification theory provides a qualitative framework for their formation, the detailed physical processes remain incompletely understood. Previous studies have shown that the bar formation timescale in isolated galaxies depends exponentially on the disk mass fraction (the so-called “Fujii relation”) and linearly on disk hotness and thickness. However, the influence of dark matter (DM) halo spin on bar formation has not been systematically investigated. In this work, we construct a suite of N -body disk–halo models with varying disk mass fractions and amounts of random motions. By introducing prograde and retrograde spins in the DM halo, we explore how halo spin modifies the established empirical relations governing bar formation timescales. We find that these relations remain valid in both prograde and retrograde halo spin models. For rapid bar formation (short timescale), the effect of halo spin is nearly negligible. In contrast, for moderately slow bar formation, prograde (retrograde) halo spin tends to accelerate (suppress) bar onset. In cases of extremely slow bar formation, halo spin introduces a stronger but more stochastic influence. These trends might arise from the exchange of angular momentum between the stellar disk and the DM halo.