arXiv subjects

Juntai Shen

Publications and source records attributed to Juntai Shen.

At least 19 recordsLinked to original sources

arXiv2026

SchwarMAX: a GPU-friendly Schwarzschild orbit-superposition modelling framework

The Schwarzschild orbit-superposition method is a highly flexible dynamical modelling tool. It constrains the mass distribution of a galaxy using line-of-sight velocity and photometric observations. However, constructing such a dynamical model of a galaxy is computationally expensive. We present SchwarMAX, a new publicly available GPU implementation of the Schwarzschild orbit-superposition method. The GPU-native code is significantly faster than other implementations, with entire model construction taking around a second on GPU A100. Using SchwarMAX, we can explore the distributions of both baryonic and dark matter in a galaxy across a high-dimensional parameter space. We demonstrate its performance using mock integrated-field spectroscopic unit data generated from an N-body simulated barred galaxy. We explore the 12-dimensional space of disc, bar and halo parameters using Markov Chain Monte Carlo. The density profiles and the bar pattern speed of the galaxy are recovered with good accuracy. We show that the code can be applied to barred galaxies across a wide range of inclination angles and can be easily extended to other stellar systems, such as elliptical and dwarf galaxies.

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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.

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arXiv2026

Galactic Stellar Halo Luminosity Function

We measure the luminosity function (LF) of the Milky Way's stellar halo, using a magnitude complete, distance limited sample of stars from $Gaia$ DR3. Stars with high transverse velocities are selected, to isolate a high purity sample of the local halo. We adopt a cutoff transverse velocity of 250$\,$km$\,$s$^{-1}$, yielding 24,471 stars, and compute the halo LF, taking into account the effects of sample selection criteria. The LF displays similar features as are found in the well-probed LF of nearby, metal-rich disk stars, showing a strong peak at an absolute magnitude of around $M_G=10$, and a flattening near $M_G\sim7$ (Wielen dip). The $Gaia$ sample yields the first measurement of the LF continuously from the dimmest main sequence halo stars (subdwarfs) at an absolute $M_G$ magnitude near 13 mag to bright giants at $M_G\sim-3$. We obtain a local stellar halo number density of $1.7\times10^{-4}$ stars$\,$pc$^{-3}$ and disk-to-halo ratio by stellar number density of 480:1. We convert the $Gaia$ $G$ band measurements for our sample stars to Johnson-Kron-Cousins $V$ band, compute the $V$-band halo LF, and compare it to previous studies published over many decades that cover a wide range of techniques used. We discuss applications of the LF to the measurement of the luminosity and stellar mass of the Milky Way halo.

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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.

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arXiv2026

Orbital classification in rotating bar potentials using an empirical proxy of the second integral of motion

We present a novel method for classifying two-dimensional orbits in rotating bar potentials, based on an empirical proxy for the second integral of motion, Calibrated Angular Momentum (CAM), which is defined as the ratio of the time-averaged angular momentum ($\overline{L_z}$) to its temporal dispersion ($σ_{L_z}$) in the corotating frame. We show that CAM is determined by the ratio of the azimuthal to radial actions (${J_ϕ}^\prime / {J_r}^\prime$) in the analytical Freeman bar model. We then construct a new parameter space defined by CAM versus the root-mean-square radius ($R_{RMS}$), and apply this framework to orbits in several representative rotating bar potentials. In the CAM-$R_{RMS}$ plane, periodic orbits generate well-defined branches separating distinct regions corresponding to different orbital families. Several of these branches enclose isolated areas that can be associated with specific orbital families, such as the the $x_2$ orbital family. We further validate the method using orbits from test-particle simulations, which show a well-ordered and non-overlapping distribution of orbital families in the CAM-$R_{RMS}$ plane. Since CAM is fundamentally linked to intrinsic orbital properties and readily applied to three-dimensional orbits in N-body simulations, our results establish the CAM-$R_{RMS}$ plane as a robust and efficient framework for orbit classification in rotating bars that complements conventional methods.

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arXiv2026

Revisiting the Radial Metallicity Gradient-Age Relation in the Milky Way's Thin and Thick Disks

Galactic disks typically exhibit a negative radial metallicity gradient, indicating faster enrichment in the inner regions. Recent studies report that this gradient becomes flatter with increasing stellar age in the Milky Way's (MW) thin disk, while the thick disk exhibits a mildly positive gradient across all ages. In this work, we revisit the metallicity gradient-age relation (MGAR) in both the thin and thick disks of the MW, using spectroscopic data from LAMOST DR8 and stellar ages calibrated with asteroseismology. Our results show a steadily flattening MGAR in the thin disk and confirm a positive gradient $\sim0.013\,\mathrm{dex\,kpc^{-1}}$ in the thick disk. The flattening in the thin disk may be caused by large-scale radial migration induced by transient spiral arms, or by a time-dependent steepening of the interstellar medium (ISM) metallicity gradient as suggested by recent FIRE2 simulations. The positive gradient in the thick disk may reflect early enrichment of the outer regions by strong feedback or starburst-driven outflows in a turbulent, gas-rich proto-disk. These findings suggest distinct chemodynamical evolution paths for the MW's thin and thick disks and provide valuable constraints for future models of Galactic chemical evolution.

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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.

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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.

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arXiv2025

Introduction to the Chinese Space Station Survey Telescope (CSST)

The Chinese Space Station Survey Telescope (CSST) is an upcoming Stage-IV sky survey telescope, distinguished by its large field of view (FoV), high image quality, and multi-band observation capabilities. It can simultaneously conduct precise measurements of the Universe by performing multi-color photometric imaging and slitless spectroscopic surveys. The CSST is equipped with five scientific instruments, i.e. Multi-band Imaging and Slitless Spectroscopy Survey Camera (SC), Multi-Channel Imager (MCI), Integral Field Spectrograph (IFS), Cool Planet Imaging Coronagraph (CPI-C), and THz Spectrometer (TS). Using these instruments, CSST is expected to make significant contributions and discoveries across various astronomical fields, including cosmology, galaxies and active galactic nuclei (AGN), the Milky Way and nearby galaxies, stars, exoplanets, Solar System objects, astrometry, and transients and variable sources. This review aims to provide a comprehensive overview of the CSST instruments, observational capabilities, data products, and scientific potential.

astro-ph.IM↗

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.

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arXiv2025

The Impact of Bar-induced Non-Circular Motions on the Measurement of Galactic Rotation Curves

We study the impact of bar-induced non-circular motions on the derivation of galactic rotation curves (RCs) using hydrodynamic simulations and observational data from the PHANGS-ALMA survey. We confirm that non-circular motions induced by a bar can significantly bias RCs derived from the conventional tilted-ring method, consistent with previous findings. The shape of the derived RC depends on the position angle difference ($Δϕ$) between the major axes of the bar and the disk in the face-on plane. For $\left|Δϕ\right|\lesssim40^\circ$, non-circular motions produce a bar-induced "dip" feature (rise-drop-rise pattern) in the derived RC, which shows higher velocities near the nuclear ring and lower velocities in the bar region compared to the true RC (${\mathrm{RC_{true}}}$). We demonstrate convincingly that such dip features are very common in the PHANGS-ALMA barred galaxies sample. Hydrodynamical simulations reveal that the "dip" feature is caused by the perpendicular orientation of the gas flows in the nuclear ring and the bar; at low $\left|Δϕ\right|$ streamlines in the nuclear ring tend to enhance $V_\mathrm{los}$, while those in the bar tend to suppress $V_\mathrm{los}$. We use a simple {\misaell} model to qualitatively explain the general trend of RCs from the tilted-ring method (${\mathrm{RC_{tilted}}}$) and the discrepancy between ${\mathrm{RC_{tilted}}}$ and ${\mathrm{RC_{true}}}$. Furthermore, we propose a straightforward method to implement a first-order correction to the RC derived from the tilted-ring method. Our study is the first to systematically discuss the bar-induced "dip" feature in the RCs of barred galaxies combining both simulations and observations.

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arXiv2024

Comparison of bar formation mechanisms I: does a tidally-induced bar rotate slower than an internally-induced bar?

Galactic bars can form via the internal bar instability or external tidal perturbations by other galaxies. We systematically compare the properties of bars formed through the two mechanisms with a series of controlled $N$-body simulations that form bars through internal or external mechanisms. We create three disk galaxy models with different dynamical ``hotness'' and evolve them in isolation and under flyby interactions. In the cold and warm disk models, where bars can form spontaneously in isolation, tidally-induced bars are promoted to a more ``advanced'' evolutionary stage. However, these bars have similar pattern speeds to those formed spontaneously within the same disk. Bars formed from both mechanisms have similar distributions in pattern speed--bar strength ($Ω_p-A_2$) space and exhibit comparable ratios of co-rotation radius to bar length (${\cal R}={R_{\mathrm {CR}}}/{R_{\mathrm {bar}}}$). Dynamical analyses suggest that the inner stellar disk loses the same amount of angular momentum, irrespective of the presence or intensity of the perturbation, which possibly explains the resemblance between tidally and spontaneously formed bars. In the hot disk model, which avoids the internal bar instability in isolation, a bar forms only under perturbations and rotates more slowly than those in the cold and warm disks. Thus, if ``tidally-induced bars'' refer exclusively to those in galaxies that are otherwise stable against bar instability, they indeed rotate slower than internally-induced ones. However, the pattern speed difference is due to the difference in the internal properties of the bar host galaxies, not the different formation mechanisms.

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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.

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arXiv2024

Orbital Support and Evolution of CX/OX Structures in Boxy/Peanut Bars

Barred galaxies exhibit boxy/peanut or X-shapes (BP/X) protruding from their disks in edge-on views. Two types of BP/X morphologies exist depending on whether the X-wings meet at the center (CX) or are off-centered (OX). Orbital studies indicate that various orbital types can generate X-shaped structures. Here, we provide a classification approach that identifies the specific orbit families responsible for generating OX and CX-shaped structures. Applying this approach to three different N-body bar models, we show that both OX and CX structures are associated with the x1 orbit family, but OX-supporting orbits possess higher angular momentum (closer to x1 orbits) than orbits in CX structures. Consequently, as the bar slows down the contribution of higher angular momentum OX-supporting orbits decreases and that of lower angular momentum orbits increases resulting in an evolution of the morphology from OX to CX. If the bar does not slow down, the shape of the BP/X structure and the fractions of OX/CX supporting orbits remain substantially unchanged. Bars that do not undergo buckling but that do slow down initially show the OX structure and are dominated by high angular momentum orbits, transitioning to a CX morphology. Bars that buckle exhibit a combination of both OX and CX supporting orbits immediately after the buckling, but become more CX dominated as their pattern speed decreases. This study demonstrates that the evolution of BP/X morphology and orbit populations strongly depends on the evolution of the bar angular momentum.

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arXiv2024

Deciphering the Kinematic Substructure of Local Dark Matter with LAMOST K Giants

Numerical simulations indicate that correlations exist between the velocity distributions of stars and dark matter (DM). We study the local DM velocity distribution based on these correlations. We select K giants from LAMOST DR8 cross-matched with {\gaia} DR3, which have robust measurements of velocity and metallicity, and separate them into the disk, halo substructure and isotropic halo components in the chemodynamical space utilizing the Gaussian Mixture Model. The substructure component is highly radially anisotropic, and possibly related to the \gaia-Enceladus-Sausage (GES) merger event, while the isotropic halo component is accreted from the earliest mergers following the Maxwell-Boltzmann distribution (Standard Halo Model, SHM). We find that the GES-like substructure contributes $\sim85\%$ of the local nondisk stars in the Solar neighbourhood, which is nearly invariant when applying different volume cuts or additional angular momentum constraints. Utilizing the metallicity-stellar mass relation and the stellar mass-halo mass relation, we find that $\sim25_{-15}^{+24}\%$ of local DM is in the kinematic substructure. Combined with the stellar distributions of nondisk components, we modify the heliocentric velocity distribution of local DM. It shifts to a lower speed with a sharper peak compared to the SHM, and updates the detection limits of DM direct detection experiments. We discuss extensively the degeneracies present in the GMM fitting and propose that more kinematic and chemical information such as $α$ abundance could help to break the degeneracy in the future. Our work confirms that the local DM velocity distribution deviates significantly from the SHM, and needs to be properly accounted for in the DM detection experiments.

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arXiv2024

The Milky Way bar potential constrained by the kinematics of SiO maser stars in BAaDE Survey

We introduce a novel method that utilizes the longitude-velocity (l-v) envelope to constrain the Milky Way (MW) bar potential. Previous work (Habing 2016) used the l-v diagram to explain the distribution of the observed high-velocity stars. We successfully reproduce their results, but find that their method is limited to only one single type of periodic orbits. In contrast, we propose that the l-v envelope provides much more comprehensive constraints. We compare the properties of test particles in the Portai et al. (2017) MW potential model (P17) with the observed SiO maser stars from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey. We find that the l-v envelope generated by the bar potential demonstrates reasonable agreement with the observational data, albeit with slight discrepancies near the Galactic center. The inconsistencies suggest that the P17 potential yields a lower central rotation curve, a slightly larger quadrupole strength, or a possibly underestimated pattern speed. We also adopt an updated version of the P17 potential with a modified central mass component (CMC) proposed by Hunter et al. (2024) (H24). The fitting of the l-v envelope suggests that the H24 potential does not completely address the existing challenges and may hint at a possible underestimation of the central bar mass. Our study demonstrates that the l-v envelope can be used as a valuable tool for constraining the Galactic potential and provides insights into the Milky Way bar potential.

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arXiv2024

Schwarzschild Modeling of Barred S0 Galaxy NGC 4371

We apply the barred Schwarzschild method developed by Tahmasebzadeh et al. (2022) to a barred S0 galaxy, NGC 4371, observed by IFU instruments from the TIMER and ATLAS3D projects. We construct the gravitational potential by combining a fixed black hole mass, a spherical dark matter halo, and stellar mass distribution deprojected from $3.6$ $μ$m S$^4$G image considering an axisymmetric disk and a triaxial bar. We independently modelled kinematic data from TIMER and ATLAS3D. Both models fit the data remarkably well. We find a consistent bar pattern speed from the two sets of models with $Ω_{\rm p} = 23.6 \pm 2.8 \hspace{.08cm} \mathrm{km \hspace{.04cm} s^{-1} \hspace{.04cm} kpc^{-1} }$ and $Ω_{\rm p} = 22.4 \pm 3.5 \hspace{.08cm} \mathrm{km \hspace{.04cm} s^{-1} \hspace{.04cm} kpc^{-1} }$, respectively. The dimensionless bar rotation parameter is determined to be $ 1.88 \pm 0.37$, indicating a likely slow bar in NGC 4371. Additionally, our model predicts a high amount of dark matter within the bar region ($M_{\rm DM}/ M_{\rm total}$ $\sim 0.51 \pm 0.06$), which, aligned with the predictions of cosmological simulations, indicates that fast bars are generally found in baryon-dominated disks. Based on the best-fitting model, we further decompose the galaxy into multiple 3D orbital structures, including a BP/X bar, a classical bulge, a nuclear disk, and a main disk. The BP/X bar is not perfectly included in the input 3D density model, but BP/X-supporting orbits are picked through the fitting to the kinematic data. This is the first time a real barred galaxy has been modelled utilizing the Schwarzschild method including a 3D bar.

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arXiv2024

Reply to Comment on "A slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp"

In this reply, we present a comprehensive analysis addressing the concerns raised by Dehnen et al. (2024) regarding our recent measurement of the disk warp precession using the `motion-picture' method (Huang et al. 2024). We carefully examine the impact of ignoring the twist of the disk warp and the so-called $R$-$τ$ correlation on the estimation of the precession rate. The results indicate that the effect is minor and does not exceed the systematic and statistical uncertainties. Using N-body+SPH simulation data, we confirm that the `motion-picture' technique is effective in measuring retrograde precession of disk warp in stellar populations younger than 170 Myr, similar to classical Cepheids. Therefore, the overall conclusions of Huang et al. (2024) remain robust.

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