arXiv subjects

Rimpei Chiba

Publications and source records attributed to Rimpei Chiba.

12 recordsLinked to original sources

arXiv2026

Tree-ring structure of Galactic bar resonance in N-body simulations

We study the structure and evolution of the galactic bar's resonant phase-space in self-consistent N-body simulations of the Milky Way, with and without perturbations from the Sagittarius dwarf galaxy. In an idealized disk evolution model in which stars are perturbed solely by a bar that spins down due to dynamical friction against the dark matter halo, it is predicted that stars trapped in the bar's corotation resonance form a characteristic `tree-ring' structure in phase space: as the resonance expands in volume while sweeping outwards, it sequentially captures surrounding stars at its surface, such that stars captured earlier in the inner disk are found preferentially near the core of the resonance. However, it has not been clear whether such a structure persists in a more realistic galactic disk subject to a variety of time-dependent perturbations, in particular those by spiral arms and passing satellite galaxies. This paper demonstrates that the predicted tree-ring structure indeed emerges in a realistic noisy environment using self-consistent N-body simulations. Despite the presence of spiral arms, encounters with the Sagittarius dwarf galaxy, as well as fluctuations in the bar's pattern speed, and not least numerical noise -- all of which drive stellar diffusion in phase space -- the tree-ring structure remains well-preserved in the slow angle-action space. Our results demonstrate that the tree-ring structure of the bar's resonance is a robust signal of the bar's spin-down and hence its discovery in the Milky Way implies the existence of a dark matter halo that removed angular momentum from the bar.

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arXiv2026

Galaxy fly-bys sustain bar-halo friction and bar slowdown in disk galaxies

Bars in disk galaxies slow down as they transfer their angular momentum to their dark matter halo via dynamical friction from near-resonant orbits. This bar-halo dynamical friction can become ineffective once phase mixing erases the phase-space gradient around the main resonances. We present fully self-consistent $N$-body simulations of a Milky Way-like disk galaxy with a single dwarf-galaxy fly-by in prograde and retrograde orbits before, during, and after bar formation. In our models, the fly-bys do not trigger a long-lived tidal bar; the bar forms on essentially the same time as in the isolated model. After the encounter, however, all perturbed models develop bars that are stronger and slower than in the isolated one. The final pattern speed depends little on the encounter time, but it does depend on the encounter direction relative to the disk rotation: prograde encounters slow the bar more than retrograde ones. The angular-momentum evolution shows that the disk loses its angular momentum and the halo gains it, consistent with bar-halo friction. By probing the particle distribution of the halo in angle-action space, we demonstrate that the isolated bar enters a metastable, saturated state with a flattened distribution in the phase space around the bar's corotation resonance, whereas a dwarf passage excites long-lived fluctuations in the halo that restore the phase-space gradients near the corotation and thereby sustain the bar-halo friction. This mechanism explains the continued slowdown and growth of bars after fly-bys. It may be relevant to the Milky Way, whose bar formed near the epoch of a major ancient accretion event, suggesting that an early encounter could have influenced the subsequent secular evolution of the bar.

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arXiv2026

Winding, Unwinding, Rewinding the Gaia Phase Spiral

The Gaia Space Satellite has transformed the field of Galactic Dynamics by collecting 6D phase space information for hundreds of millions of stars. In 2018, it enabled the discovery of the Gaia Phase Spiral (Antoja et al., 2018), a clear signal in the vertical motion of the stars that reveals how far from equilibrium the Galactic disk is. Seven years after the discovery of this structure, a workshop dedicated to the Phase Spiral took place at the Lorentz Center. Workshop participants summarized the current state of knowledge about the Phase Spiral and identified open questions and key areas to continue progressing in understanding the origin of the Phase Spiral and the physics governing the response of the disk to perturbations. Here, we aim to summarize the content and discussions of this workshop, share the resources that have been produced at this workshop with the broader community, and invite interested individuals to join on the projects that started.

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arXiv2025

Galactic echoes

Gaia has revealed a variety of substructures in the phase space of stars in the Solar neighborhood, including the vertical `Snail' in $(z,v_z)$ space. Such substructures are often interpreted as the incompletely phase-mixed response of the disc stars to a single perturbation, such as an impulsive encounter with a satellite galaxy. In this paper we consider the possibility that such structures contain manifestations of phase space echoes. First established in plasma physics in the 1960s, echoes arise when a collisionless system is perturbed twice: the macroscopic responses to both perturbations mix to small scales in phase space, whereupon they couple nonlinearly, producing a third macroscopic `echo' response without the need for a third perturbation. We derive the galactic analogue of the plasma echo theory using angle-action variables and apply it to a one-dimensional model of vertical motion in the Milky Way. We verify the predicted echo behavior using idealized test particle simulations, both with and without the inclusion of diffusion through orbital scattering off molecular clouds. While we conclude that the Gaia Snail itself is unlikely a (pure) echo effect, the basic physics we uncover is sufficiently generic that we expect phase-space echoes to be common in disc galaxies.

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arXiv2025

Galaxy Zoo CEERS: Bar fractions up to z~4.0

We study the evolution of the bar fraction in disc galaxies between $0.5 < z < 4.0$ using multi-band coloured images from JWST CEERS. These images were classified by citizen scientists in a new phase of the Galaxy Zoo project called GZ CEERS. Citizen scientists were asked whether a strong or weak bar was visible in the host galaxy. After considering multiple corrections for observational biases, we find that the bar fraction decreases with redshift in our volume-limited sample (n = 398); from $25^{+6}_{-4}$% at $0.5 < z < 1.0$ to $3^{+6}_{-1}$% at $3.0 < z < 4.0$. However, we argue it is appropriate to interpret these fractions as lower limits. Disentangling real changes in the bar fraction from detection biases remains challenging. Nevertheless, we find a significant number of bars up to $z = 2.5$. This implies that discs are dynamically cool or baryon-dominated, enabling them to host bars. This also suggests that bar-driven secular evolution likely plays an important role at higher redshifts. When we distinguish between strong and weak bars, we find that the weak bar fraction decreases with increasing redshift. In contrast, the strong bar fraction is constant between $0.5 < z < 2.5$. This implies that the strong bars found in this work are robust long-lived structures, unless the rate of bar destruction is similar to the rate of bar formation. Finally, our results are consistent with disc instabilities being the dominant mode of bar formation at lower redshifts, while bar formation through interactions and mergers is more common at higher redshifts.

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arXiv2025

Origin of the two-armed vertical phase-spiral in the inner Galactic disk

Gaia recently revealed a two-armed spiral pattern in the vertical phase-space distribution of the inner Galactic disk (guiding radius $R_\textrm{g} \sim 6.2$ kpc), indicating that some non-adiabatic perturbation symmetric about the mid-plane is driving the inner disk out of equilibrium. The non-axisymmetric structures in the disk (e.g., the bar or spiral arms) have been suspected to be the major source for such a perturbation. However, both the lifetime and the period of these internal perturbations are typically longer than the period at which stars oscillate vertically, implying that the perturbation is generally adiabatic. This issue is particularly pronounced in the inner Galaxy, where the vertical oscillation period is shorter and therefore adiabatically shielded more than the outer disk. We show that two-armed phase spirals can naturally form in the inner disk if there is a vertical resonance that breaks the adiabaticity; otherwise, their formation requires a perturber with an unrealistically short lifetime. We predict analytically and confirm with simulations that a steadily rotating (non-winding) two-armed phase spiral forms near the resonance when stars are subject to both periodic perturbations (e.g., by spiral arms) and stochastic perturbations (e.g., by giant molecular clouds). Due to the presence of multiple resonances, the vertical phase-space exhibits several local phase spirals that rotate steadily at distinct frequencies, together forming a global phase spiral that evolves over time. Our results demonstrate that, contrary to earlier predictions, the formation of the two-armed phase spiral does not require transient perturbations with lifetimes shorter than the vertical oscillation period.

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arXiv2024

Exploring the impact of a rapidly decelerating bar on transforming bulge orbits into disc-like orbits

The most metal-poor tail of the Milky Way ([Fe/H] $\leq$ $-$2.5) contains a population of stars with very prograde planar orbits, which is puzzling in both their origin and evolution. A possible scenario is that they are shepherded by the bar from the inner Galaxy, where many of the old and low-metallicity stars in the Galaxy are located. To investigate this scenario, we use test-particle simulations with an axisymmetric background potential plus a central bar model. The test particles are generated by an extended distribution function (EDF) model based on the observational constraints of bulge stars. According to the simulation results, a bar with constant pattern speed cannot help bring stars from the bulge to the solar vicinity. In contrast, when the model includes a rapidly decelerating bar, some bulge stars can gain rotation and move outwards as they are trapped in the co-rotation regions of the bar. The resulting distribution of shepherded stars heavily depends on the present-day azimuthal angle between the bar and the Sun. The majority of the low-metallicity bulge stars driven outwards are distributed in the fourth quadrant of the Galaxy with respect to the Sun, and about 10$\%$ of them are within 6 kpc from us. Our experiments indicate that the decelerating bar perturbation can be a contributing process to explain part of the most metal-poor stars with prograde planar orbits seen in the Solar neighborhood but is unlikely to be the dominant one.

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arXiv2024

Origin of reduced dynamical friction by dark matter halos with net prograde rotation

We provide an explanation for the reduced dynamical friction on galactic bars in spinning dark matter halos. Earlier work based on linear theory predicted an increase in dynamical friction when dark halos have a net forward rotation, because prograde orbits couple to bars with greater strength than retrograde orbits. Subsequent numerical studies, however, found the opposite trend: dynamical friction weakens with increasing spin of the halo. We revisit this problem and demonstrate that linear theory in fact correctly predicts a reduced torque in forward-rotating halos. We show that shifting the halo mass from retrograde to prograde phase space generates a positive gradient in the distribution function near the origin of the z-angular momentum (Lz=0), which results in a resonant transfer of Lz to the bar, making the net dynamical friction weaker. While this effect is subdominant for the major resonances, including the corotation resonance, it leads to a significant positive torque on the bar for the series of direct radial resonances, as these resonances are strongest at Lz=0. The overall dynamical friction from spinning halos is shown to decrease with the halo's spin, in agreement with the secular behavior of N-body simulations. We validate our linear calculation by computing the nonlinear torque from individual resonances using the angle-averaged Hamiltonian.

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arXiv2023

Dynamical friction and feedback on galactic bars in the general fast-slow regime

Current theories of dynamical friction on galactic bars are based either on linear perturbation theory, which is valid only in the fast limit where the bar changes its pattern speed rapidly, or on adiabatic theory, which is applicable only in the slow limit where the bar's pattern speed is near-constant. In this paper, we study dynamical friction on galactic bars spinning down at an arbitrary speed, seamlessly connecting the fast and slow limits. We treat the bar-halo interaction as a restricted $N$-body problem and solve the collisionless Boltzmann equation using the fast-angle-averaged Hamiltonian. The phase-space distribution and density wakes predicted by our averaged model are in excellent agreement with full 3D simulations. In the slow regime where resonant trapping occurs, we show that, in addition to the frictional torque, angular momentum is transferred directly due to the migration of the trapped phase-space: trapped orbits comoving with the resonance typically gain angular momentum, while untrapped orbits leaping over the trapped island lose angular momentum. Due to the negative gradient in the distribution function, gainers typically outnumber the losers, resulting in a net negative torque on the perturber. Part of this torque due to the untrapped orbits was already identified by Tremaine & Weinberg who named the phenomenon dynamical feedback. Here, we derive the complete formula for dynamical feedback, accounting for both trapped and untrapped orbits. Using our revised formula, we show that dynamical feedback can account for up to $30\%$ of the total torque on the Milky Way's bar.

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arXiv2022

Oscillating dynamical friction on galactic bars by trapped dark matter

The dynamic evolution of galactic bars in standard $Λ$CDM models is dominated by angular momentum loss to the dark matter haloes via dynamical friction. Traditional approximations to dynamical friction are formulated using the linearized collisionless Boltzmann equation and have been shown to be valid in the fast limit, i.e. for rapidly slowing bars. However, the linear assumption breaks down within a few dynamical periods for typical slowly evolving bars, which trap a significant amount of disc stars and dark matter in resonances. Recent observations of the Galactic bar imply this slow regime at the main bar resonances. We formulate the time-dependent dynamical friction in the slow limit and explore its mechanism in the general slow regime with test-particle simulations. Here, angular momentum exchange is dominated by resonantly trapped orbits which slowly librate around the resonances. In typical equilibrium haloes, the initial phase-space density within the trapped zone is higher at lower angular momentum. Since the libration frequency falls towards the separatrix, this density contrast winds up into a phase-space spiral, resulting in a dynamical friction that oscillates with $\sim$Gyr periods and damps over secular timescales. We quantify the long-term behaviour of this torque with secular perturbation theory, and predict two observable consequences: i) The phase-space spirals may be detectable in the stellar disc where the number of windings encodes the age of the bar. ii) The torque causes weak oscillations in the bar's pattern speed, overlaying the overall slowdown -- while not discussed, this feature is visible in previous simulations.

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arXiv2021

Tree-ring structure of Galactic bar resonance

Galaxy models have long predicted that galactic bars slow down by losing angular momentum to their postulated dark haloes. When the bar slows down, resonance sweeps radially outwards through the galactic disc while growing in volume, thereby sequentially capturing new stars at its surface/separatrix. Since trapped stars conserve their action of libration, which measures the relative distance to the resonance centre, the order of capturing is preserved: the surface of the resonance is dominated by stars captured recently at large radius, while the core of the resonance is occupied by stars trapped early at small radius. The slow-down of the bar thus results in a rising mean metallicity of trapped stars from the surface towards the centre of the resonance as the Galaxy's metallicity declines towards large radii. This argument, when applied to Solar neighbourhood stars, allows a novel precision measurement of the bar's current pattern speed $Ω_p = 35.5 \pm 0.8$ km/s/kpc, placing the corotation radius at $R_{CR} = 6.6 \pm 0.2$ kpc. With this pattern speed, the corotation resonance precisely fits the Hercules stream in agreement with kinematics. Beyond corroborating the slow bar theory, this measurement manifests the deceleration of the bar of more than 24% since its formation and thus the angular momentum transfer to the dark halo by dynamical friction. The measurement therefore supports the existence of a standard dark-matter halo rather than alternative models of gravity.

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arXiv2020

Resonance sweeping by a decelerating Galactic bar

We provide the first quantitative evidence for the deceleration of the Galactic bar from local stellar kinematics in agreement with dynamical friction by a typical dark matter halo. The kinematic response of the stellar disk to a decelerating bar is studied using secular perturbation theory and test particle simulations. We show that the velocity distribution at any point in the disk affected by a naturally slowing bar is qualitatively different from that perturbed by a steadily rotating bar with the same current pattern speed $Ω_{\rm p}$ and amplitude. When the bar slows down, its resonances sweep through phase space, trapping and dragging along a portion of previously free orbits. This enhances occupation on resonances, but also changes the distribution of stars within the resonance. Due to the accumulation of orbits near the boundary of the resonance, the decelerating bar model reproduces with its corotation resonance the offset and strength of the Hercules stream in the local $v_R$-$v_φ$ plane and the double-peaked structure of mean $v_R$ in the $L_z$-$φ$ plane. At resonances other than the corotation, resonant dragging by a slowing bar is associated with a continuing increase in radial action, leading to multiple resonance ridges in the action plane as identified in the Gaia data. This work shows models using a constant bar pattern speed likely lead to qualitatively wrong conclusions. Most importantly we provide a quantitative estimate of the current slowing rate of the bar $dΩ_{\rm p}/dt = (-4.5 \pm 1.4)~{\rm km} {\rm s}^{-1} {\rm kpc}^{-1} {\rm Gyr}^{-1}$ with additional systematic uncertainty arising from unmodeled impacts of e.g. spiral arms.

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