arXiv subjects

Bin-Hui Chen

Publications and source records attributed to Bin-Hui Chen.

7 recordsLinked to original sources

arXiv2026

Comparison of Bar Formation Mechanisms. IIIA. The role of classical bulges in spontaneous bar formation

We run a suite of $N$-body simulations to investigate how classical bulges affect bar formation and properties under the internal formation mechanism. We incorporate bulges of varying mass and compactness into disk galaxy models and evolve them in isolation to examine the resulting bar pattern speeds and growth timescales. A more massive/compact bulge increases the Toomre $Q$ stability parameter and the circular velocity in the central region, while decreasing the disk mass fraction. It therefore delays the onset of bar formation and increases the bar growth timescale; sufficiently strong bulges can suppress bar formation entirely. During the formation stage, bars exhibit higher initial pattern speeds and faster deceleration rates when the bulges become more massive or compact. This faster deceleration persists after the bar buckling phase, leading to slower-rotating bars in the secular growth stage. However, when the bulge's "diluting" effect on the measured bar strength is removed or reduced, all bars within the same disk share similar distributions in the pattern speed-bar strength ($Ω_p$-$A_2$) space during the secular growth stage. They also show comparable ratios of the co-rotation radius to the bar length ($\mathcal{R}=R_{\mathrm{CR}}/R_{\mathrm {bar}}$) in this stage. These results suggest that the bulge's influence on the pattern speed is more significant during the bar formation stage, while in the secular growth stage, the bulge's effect may be less important, and the disk component dominates the pattern speed evolution.

astro-ph.GA↗

arXiv2026

Bar Formation During a Gaia-Sausage-Enceladus-like Merger Event

Bars are among the most prominent galactic structures, yet their formation mechanisms remain incompletely understood. They can form either internally, via dynamical instabilities, or externally, triggered by interactions with other galaxies. The impact of mergers on bar formation and survival, however, has not been thoroughly investigated. To explore the influence of mergers on bars, we construct a suite of \textit{N}-body merger pairs where a Gaia-Sausage-Enceladus-like radially biased satellite disk galaxy merges with a central disk galaxy during its bar formation. With the central galaxy fixed, the satellite varies in merger parameters: the mass ratio $m/M$ relative to the central galaxy, the impact parameter $b$, and the orbital inclination angle $θ_i$ relative to the central disk. We find that the bar survival probability decreases with increasing $m/M$. Mergers with $m/M\lesssim1/10$ generally preserve the forming bar, whereas those with ${m/M}\geq1/2$ tend to destroy it, producing more early-type-like remnants. For intermediate mass ratios ($1/5 \leq m/M \leq 1/3$), several models yield ``weakening bars'', in which the bar survives the merger but gradually decays during subsequent secular evolution, possibly due to interactions between nested double bars formed from merger debris. In contrast to $m/M$, $b$ and $θ_i$ have only secondary and stochastic effects on bar survival. The different influences of these three merger parameters can be naturally explained by the tidal force exerted by the satellite on the forming bar, which tends to weaken the bar when the satellite crosses it nearly perpendicular to its major axis.

astro-ph.GA↗

arXiv2025

Dependency of the Bar Formation Timescale On The Halo Spin

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 halo spin on bar formation has not been systematically investigated. In this work, we construct a suite of $N$-body models of disk and halo with varying disk mass fractions and amounts of random motions. By introducing prograde and retrograde spins in the dark matter 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 dark matter halo.

astro-ph.GA↗

arXiv2025

The Dependency of Bar Formation Timescale on Disk Mass Fraction, Toomre $Q$, and Scale Height

Bars are one of the most prominent galactic structures. The classical swing-amplification theory can qualitatively describe the spontaneous bar instability of stellar disks. Still, it cannot quantify the bar formation process or explain why some disk galaxies do not have a bar. Recent studies found that the bar formation timescale depends exponentially on the disk mass fraction of the host galaxy (dubbed as "Fujii relation"), but they only explored a limited parameter space, where the physical effects of Toomre $Q$ (local disk stability parameter) and disk scale height of the host galaxies are not fully explored. In this work, we check the robustness of the Fujii relation in a higher-dimensional parameter space of disk mass fraction, Toomre $Q$, and scale height. We find that the Fujii relation holds for disk galaxies with physically reasonable Toomre $Q$ and scale height. Furthermore, the bar formation timescale also approximately linearly depends on both Toomre $Q$ and scale height, with a more prolonged bar formation in a hotter or thicker disk. We propose an empirical relation to combine the dependency of the bar formation timescale on the three parameters. Based on the empirical relation and recent observations, we estimate that the bar formation timescale in pure stellar disks ranges from $0.20_{-0.06}^{+0.09}~\mathrm{Gyr}$ to $12.20_{-2.80}^{+3.37}~\mathrm{Gyr}$ or even significantly beyond the Hubble timescale in some extreme cases.

astro-ph.GA↗

arXiv2025

Comparison of Bar Formation Mechanisms. II. Does a Tidally Induced Bar Grow Faster Than an Internally Developed Bar?

Bar structures can form internally due to the instability of their host galaxies or externally due to perturbations from other galaxies. We systematically quantify the growth timescales ($τ_\mathrm{bar}$) of bars formed through these two mechanisms with a series of controlled $N$-body simulations. In galaxies susceptible to bar instability, tidally induced bars display $τ_\mathrm{bar}$ values comparable to those of internally developed bars within the same disk. Tidal perturbations promote(delay) bar formation by advancing(postponing) its onset, but the growth rate of the bar structure remains largely unchanged. In these interaction scenarios, the bar formation is still driven primarily by the galaxy's internal nature, which remains unaffected by tidal perturbations. As the external perturbation wave reaches the galaxy's center, it evokes a "seed bar" that is then swing amplified. In this scenario, the onset of bar formation is advanced. Conversely, bar formation may be delayed if the external perturbation wave is out of phase with the preexisting spontaneously developed "seed bar", which causes destructive interference and limits the bar growth. In the hot disk model that resists bar formation in isolation, the $τ_\mathrm{bar}$ of the tidally forced bar correlates with the strength of the perturbation. The bar growth in this model deviates from an exponential profile and is better described by a linear function. The varied $τ_\mathrm{bar}$ and the preference for linear growth contrast with bars formed in galaxies inherently susceptible to bar instability. These tidally forced bars may not adhere to the swing amplification mechanism that predicts exponential bar growth.

astro-ph.GA↗

arXiv2024

Dynamical Origin of the Vertical Metallicity Gradient of the Milky Way Bulge

A vertical metallicity gradient in the Milky Way bulge is well-established. Yet, its origin has not been fully understood under the Galactic secular evolution scenario. We construct single-disk and triple-disk $N$-body models with an initial radial metallicity gradient for each disk. These models generate a vertical metallicity gradient through a ``two-step heating" mechanism: first the outer, metal-poor particles move inward via the bar instability and subsequently undergo more significant vertical heating during the buckling instability, so they end up at greater vertical height. The ``two-step heating" mechanism nearly linearly transforms the radial metallicity gradients in precursor disks into vertical metallicity gradients. Comparing the models with a triple-disk model tagged with radially independent Gaussian metallicity, we find that, despite certain limitations, the ``two-step heating" mechanism is still important in shaping the Galactic vertical metallicity gradient. If the bar and buckling instabilities contributed to the formation of boxy/peanut-shaped bulges, then the ``two-step heating" mechanism is inevitable in the secular evolution of a boxy/peanut-shaped bulge.

astro-ph.GA↗

arXiv2022

Metallicity Properties of the Galactic Bulge Stars Near and Far: Expectations from the Auriga Simulation

Using the high resolution Milky Way-like model from Auriga simulation we study the chemical properties of the Galactic bulge, focusing on the metallicity difference between stars on the near side (in front of the Galactic center) and the far side (behind the Galactic center). In general, along certain sight lines the near side is more metal-rich than the far side, consistent with the negative vertical metallicity gradient of the disk, since the far side is located higher above the disk plane than the near side. However, at the region $l<0^\circ$ and $|b|\le6^\circ$, the near side is even more metal-poor than the far side, and their difference changes with the Galactic longitude. This is mainly due to the fact that stars around the minor axis of the bar are more metal-poor than those around the major axis. Since the bar is tilted, in the negative longitude region, the near side is mainly contributed by stars close to the minor axis region than the far side to result in such metallicity difference. We extract stars in the X-shape structure by identifying the overdensities in the near and far sides. Their metallicity properties are consistent with the results of the whole Galactic bulge. The boxy/peanut-shaped bulge can naturally explain the metallicity difference of the double red clump stars in observation. There is no need to involve a classical bulge component with different stellar populations.

astro-ph.GA↗