arXiv subjects

Juntai Shen

Publications and source records attributed to Juntai Shen.

At least 37 records · Page 2Linked to original sources

arXiv2024

A slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp

The shape of the dark matter (DM) halo is key to understanding the hierarchical formation of the Galaxy. Despite extensive efforts in recent decades, however, its shape remains a matter of debate, with suggestions ranging from strongly oblate to prolate. Here, we present a new constraint on its present shape by directly measuring the evolution of the Galactic disk warp with time, as traced by accurate distance estimates and precise age determinations for about 2,600 classical Cepheids. We show that the Galactic warp is mildly precessing in a retrograde direction at a rate of $ω= -2.1 \pm 0.5 ({\rm statistical}) \pm 0.6 ({\rm systematic})$ km s$^{-1}$ kpc$^{-1}$ for the outer disk over the Galactocentric radius [$7.5, 25$] kpc, decreasing with radius. This constrains the shape of the DM halo to be slightly oblate with a flattening (minor axis to major axis ratio) in the range $0.84 \le q_Φ \le 0.96$. Given the young nature of the disk warp traced by Cepheids (less than 200 Myr), our approach directly measures the shape of the present-day DM halo. This measurement, combined with other measurements from older tracers, could provide vital constraints on the evolution of the DM halo and the assembly history of the Galaxy.

astro-ph.GA↗

arXiv2024

Importance of Initial Condition on Bar Secular Evolution: Role of Halo Angular Momentum Distribution Discontinuity

The dark matter halo properties, for example, mass, spin and concentration play a significant role in the formation and evolution of bars in disk galaxies. This study highlights the importance of a new parameter: the dark matter halo angular momentum distribution in the central region of disk. We experiment with N-body galaxy models having a disk and dark matter similar to Milky Way-type galaxies. In these models, we vary the discontinuity of the angular momentum distribution of the halo (the total spin is the same for all models). Our N-body experiments suggest that bar forms in all models after a few Gyr of disk evolution. However, in the secular evolution of the bar, as we evolve these models until 9.78 Gyr, the bar gains its strength in the model with the most continuous halo angular momentum distribution, and the bar loses strength for the most discontinuous halo angular momentum distribution. The secular evolution of the bar suggests that box/peanut/x-shaped bulges similar to those found in the Milky Way disk should be more pronounced in halos with continuous halo angular momentum distributions. This study demonstrates the importance of the initial condition setup of galaxy systems, namely the discontinuity in the dark matter halo angular momentum distribution for a given density distribution, on the bar secular evolution in the disk galaxy simulations. Further, this study helps reconcile the conflicting results of bar secular evolution in a high-spinning halo of the recent literature.

astro-ph.GA↗

arXiv2024

Insight into the Galactic Bulge Chemodynamical Properties from Gaia DR3

We explore the chemodynamical properties of the Galaxy in the azimuthal velocity $V_ϕ$ and metallicity [Fe/H] space using red giant stars from Gaia Data Release 3. The row-normalized $V_ϕ$-[Fe/H] maps form a coherent sequence from the bulge to the outer disk, clearly revealing the thin/thick disk and the Splash. The metal-rich stars display bar-like kinematics while the metal-poor stars show dispersion-dominated kinematics. The intermediate-metallicity population ($-1<$[Fe/H]$<-0.4$) can be separated into two populations, one that is bar-like, i.e. dynamically cold ($σ_{V_R}\sim80$ $\rm km\ s^{-1}$) and fast rotating ($V_ϕ\gtrsim100$ $\rm km\ s^{-1}$), and the Splash, which is dynamically hot ($σ_{V_R}\sim110$ $\rm km\ s^{-1}$) and slow rotating ($V_ϕ\lesssim100$ $\rm km\ s^{-1}$). We compare the observations in the bulge region with an Auriga simulation where the last major merger event occurred $\sim10$ Gyr ago: only stars born around the time of the merger reveal a Splash-like feature in the $V_ϕ$-[Fe/H] space, suggesting that the Splash is likely merger-induced, predominantly made-up of heated disk stars and the starburst associated with the last major merger. Since the Splash formed from the proto-disk, its lower metallicity limit coincides with that of the thick disk. The bar formed later from the dynamically hot disk with [Fe/H] $>-1$ dex, with the Splash not participating in the bar formation and growth. Moreover, with a set of isolated evolving $N$-body disk simulations, we confirm that a non-rotating classical bulge can be spun up by the bar and develop cylindrical rotation, consistent with the observation for the metal-poor stars.

astro-ph.GA↗

arXiv2023

Bar-driven Gas Dynamics of M31

The large-scale gaseous shocks in the bulge of M31 can be naturally explained by a rotating stellar bar. We use gas dynamical models to provide an independent measurement of the bar pattern speed in M31. The gravitational potentials of our simulations are from a set of made-to-measure models constrained by stellar photometry and kinematics. If the inclination of the gas disk is fixed at $i = 77^{\circ}$, we find that a low pattern speed of $16-20\;\rm km\;s^{-1}\;kpc^{-1}$ is needed to match the observed position and amplitude of the shock features, as shock positions are too close to the bar major axis in high $Ω_{b}$ models. The pattern speed can increase to $20-30\;\rm km\;s^{-1}\;kpc^{-1}$ if the inner gas disk has a slightly smaller inclination angle compared with the outer one. Including sub-grid physics such as star formation and stellar feedback has minor effects on the shock amplitude, and does not change the shock position significantly. If the inner gas disk is allowed to follow a varying inclination similar to the HI and ionized gas observations, the gas models with a pattern speed of $38\;\rm km\;s^{-1}\;kpc^{-1}$, which is consistent with stellar-dynamical models, can match both the shock features and the central gas features.

astro-ph.GA↗

arXiv2023

Measuring the Milky Way Vertical Potential with the Phase Snail in a Model Independent Way

The vertical phase-space spiral (snail) is a direct sign of dis-equilibrium of Milky Way's disc. Nevertheless, the wrapping of the phase snail contains information of the vertical potential. We propose a novel method to measure the vertical potential utilizing the intersections between the snail and $z$/$V_{z}$ axes, for which we know the maximum vertical heights ($Z_{max}$) or the maximum vertical velocities ($V_{z,max}$). Using a refined linear interpolation method, we directly obtain $(Z_{max},\ \frac{1}{2}V_{z,max}^{2})$ for these snail intersections to constrain the vertical potential profile empirically. Our method is model independent since no assumptions about the snail shape or the vertical potential have been made. Although the snail binned by the guiding center radius ($R_{g}$) is more prominent, it traces a vertical potential shallower than that of the snail binned by the same Galactocentric radius ($R$). We apply an empirical method to correct this difference. We measure the snail intersections in several $R_{g}$ bins within $7.5< R_{g} < 11.0$ kpc for Gaia DR3, and apply the interpolation method to deduce the potential values at several vertical heights. The potential at the snail intersections, as well as the following mass modeling are consistent with the popular Milky Way potentials in the literature. For the $R_{g}$-binned phase snail in the Solar neighborhood, the mass modeling indicates a local dark matter density of $ρ_{\rm dm}= 0.0150\pm0.0031$ $\rm M_{\odot}\,pc^{-3}$, consistent with previous works. Our method could be applied to larger radial ranges in future works, to provide independent and stronger constraints on the Milky Way's potential.

astro-ph.GA↗

arXiv2023

How Nested Bars Enhance, Modulate, and are Destroyed by Gas Inflows

Gas flows in the presence of two independently-rotating nested bars remain not fully understood, which is likely to play an important role in fueling the central black hole. We use high-resolution hydrodynamical simulations with detailed models of subgrid physics to study this problem. Our results show that the inner bar in double-barred galaxies can help drive gas flow from the nuclear ring to the center. In contrast, gas inflow usually stalls at the nuclear ring in single-barred galaxies. The inner bar causes a quasi-periodic inflow with a frequency determined by the difference between the two bar pattern speeds. We find that the star formation rate is higher in the model with two bars than in that with one bar. The inner bar in our model gradually weakens and dissolves due to gas inflow over a few billion years. Star formation produces metal-rich/$α$-poor stars which slows the weakening of the inner bar, but does not halt its eventual decay. We also present a qualitative comparison of the gas morphology and kinematics in our simulations with those of observed double-barred galaxies.

astro-ph.GA↗

arXiv2022

Orbit-Superposition Dynamical Modeling of Barred Galaxies

Barred structures are important in understanding galaxy evolution, but they were not included explicitly in most dynamical models for nearby galaxies due to their complicated morphological and kinematic properties. We modify the triaxial orbit-superposition Schwarzschild implementation by Van den Bosch et al. (2008) to include barred structures explicitly. The gravitational potential is a combination of a spherical dark matter halo and stellar mass; with the 3D stellar density distribution de-projected from the observed 2D image using a two-component de-projection method, including an axisymmetric disk and a triaxial barred bulge. We consider figure rotation of the galaxy with the bar pattern speed as a free parameter. We validate the method by applying it to a mock galaxy with IFU data created from an N-body simulation with a boxy/peanut or X-shaped bar. Our model fits the observed 2D surface density and all kinematic features well. The bar pattern speed is recovered well with a relative uncertainty smaller than 10%. Based on the internal stellar orbit distribution of the model, we decompose the galaxy into an X-shaped bar, a boxy bulge, a vertically extended structure and a disk, and demonstrate that our model recovers these structures generally well, similar to the true structures in the N-body simulation. Our method provides a realistic way of modelling the bar structure explicitly for nearby barred galaxies with IFU observations.

astro-ph.GA↗

arXiv2022

Effects of Inner Halo Angular Momentum on the Peanut/X-shapes of Bars

Cosmological simulations show that dark matter halos surrounding baryonic disks have a wide range of angular momenta, measured by the spin parameter ($λ$). In this study, we bring out the importance of inner angular momentum($<$30 kpc), measured in terms of the halo spin parameter, on the secular evolution of the bar using N-body simulations. We have varied the halo spin parameter $λ$ from 0 to 0.1 for co-rotating (prograde) spinning halos and one counter-rotating (retrograde) halo spin ($λ$=-0.1) with respect to the disk. We report that as the halo spin increases, the buckling is also triggered earlier and is followed by a second buckling phase in high-spin halo models. The timescale for the second buckling is significantly longer than the first buckling. We find that bar strength does not reduce significantly after the buckling in all of our models, which provides new insights about the role of inner halo angular momentum, unlike previous studies. Also, the buckled bar can still transfer significant angular momentum to the halo in the secular evolution phase, but it reduces with increasing halo spin. In the secular evolution phase, the bar strength increases and saturates to nearly equal values for all the models irrespective of halo spin and the sense of rotation with respect to the disk. The final boxy/peanut shape is more pronounced ($\sim$20 $\%$) in high spin halos having higher angular momentum in the inner region compared to non-rotating halos. We explain our results with angular momentum exchanges between the disk and halo.

astro-ph.GA↗

arXiv2022

The North/South Asymmetry of the Galaxy: Possible Connection to the Vertical Phase Space Snail

The Galaxy is found to be in disequilibrium based on recent findings of the North/South (N/S) asymmetry and the phase mixing signatures, such as a phase spiral (snail) structure in the vertical phase space ($z-V_{z}$). We show that the N/S asymmetry in a tracer population of dwarfs may be quantitatively modeled with a simple phase snail model superimposed on a smooth equilibrium background. As the phase snail intersects with the $z$ axis, the number density is enhanced, and the velocity dispersion ($σ_{z}$) is decreased relative to the other side of the Galactic plane. Fitting only to the observed asymmetric N/S $σ_{z}$ profiles, we obtain reasonable parameters for the phase space snail and the potential utilized in modeling the background, despite the complex dependence of the model on the potential parameters and the significant selection effects of the data. Both the snail shape and the N/S number density difference given by our best-fit model are consistent with previous observations. The equilibrium background implies a local dark matter density of $0.0151^{+0.0050}_{-0.0051}$ ${\rm M}_{\odot}\,{\rm pc}^{-3}$. The vertical bulk motion of our model is similar to the observation, but with a $\sim$1.2 $\rm km\,s^{-1}$ shift. Our work demonstrates the strong correlation between the phase space snail and the N/S asymmetry. Future observational constraints will facilitate more comprehensive snail models to unravel the Milky Way potential and the perturbation history encoded in the snail feature.

astro-ph.GA↗

arXiv2022

Large-scale Hydrodynamical Shocks as the Smoking Gun Evidence for a Bar in M31

The formation and evolutionary history of M31 are closely related to its dynamical structures, which remain unclear due to its high inclination. Gas kinematics could provide crucial evidence for the existence of a rotating bar in M31. Using the position-velocity diagram of [OIII] and HI, we are able to identify clear sharp velocity jump (shock) features with a typical amplitude over 100 km/s in the central region of M31 (4.6 kpc X 2.3 kpc, or 20 arcmin X 10 arcmin). We also simulate gas morphology and kinematics in barred M31 potentials and find that the bar-induced shocks can produce velocity jumps similar to those in [OIII]. The identified shock features in both [OIII] and HI are broadly consistent, and they are found mainly on the leading sides of the bar/bulge, following a hallmark pattern expected from the bar-driven gas inflow. Shock features on the far side of the disk are clearer than those on the near side, possibly due to limited data coverage on the near side, as well as obscuration by the warped gas and dust layers. Further hydrodynamical simulations with more sophisticated physics are desired to fully understand the observed gas features and to better constrain the parameters of the bar in M31.

astro-ph.GA↗

arXiv2022

Milky Way Mass with K Giants and BHB Stars Using LAMOST, SDSS/SEGUE, and Gaia: 3D Spherical Jeans Equation and Tracer Mass Estimator

We measure the enclosed Milky Way mass profile to Galactocentric distances of $\sim70$ and $\sim50$ kpc using the smooth, diffuse stellar halo samples of Bird et al. The samples are LAMOST and SDSS/SEGUE K giants (KG) and SDSS/SEGUE blue horizontal branch (BHB) stars with accurate metallicities. The 3D kinematics are available through LAMOST and SDSS/SEGUE distances and radial velocities and {\it Gaia} DR2 proper motions. Two methods are used to estimate the enclosed mass: 3D spherical Jeans equation and Evans et al. tracer mass estimator (TME). We remove substructure via the Xue et al. method based on integrals of motion. We evaluate the uncertainties on our estimates due to random sampling noise, systematic distance errors, the adopted density profile, and non-virialization and non-spherical effects of the halo. The tracer density profile remains a limiting systematic in our mass estimates, although within these limits we find reasonable agreement across the different samples and the methods applied. Out to $\sim70$ and $\sim50$ kpc, the Jeans method yields total enclosed masses of $4.3\pm0.95$ (random) $\pm0.6$ (systematic) $\times10^{11}$ M$_\odot$ and $4.1\pm1.2$ (random) $\pm0.6$ (systematic) $\times10^{11}$ M$_\odot$ for the KG and BHB stars, respectively. For the KG and BHB samples we find a dark matter virial mass of $M_{200}=0.55^{+0.15}_{-0.11}$ (random) $\pm0.083$ (systematic) $\times10^{12}$ M$_\odot$ and $M_{200}=1.00^{+0.67}_{-0.33}$ (random) $\pm0.15$ (systematic) $\times10^{12}$ M$_\odot$, respectively.

astro-ph.GA↗

arXiv2022

Blanco DECam Bulge Survey (BDBS) IV: Metallicity Distributions and Bulge Structure from 2.6 Million Red Clump Stars

We present photometric metallicity measurements for a sample of 2.6 million bulge red clump stars extracted from the Blanco DECam Bulge Survey (BDBS). Similar to previous studies, we find that the bulge exhibits a strong vertical metallicity gradient, and that at least two peaks in the metallicity distribution functions appear at b < -5. We can discern a metal-poor ([Fe/H] ~ -0.3) and metal-rich ([Fe/H] ~ +0.2) abundance distribution that each show clear systematic trends with latitude, and may be best understood by changes in the bulge's star formation/enrichment processes. Both groups exhibit asymmetric tails, and as a result we argue that the proximity of a star to either peak in [Fe/H] space is not necessarily an affirmation of group membership. The metal-poor peak shifts to lower [Fe/H] values at larger distances from the plane while the metal-rich tail truncates. Close to the plane, the metal-rich tail appears broader along the minor axis than in off-axis fields. We also posit that the bulge has two metal-poor populations -- one that belongs to the metal-poor tail of the low latitude and predominantly metal-rich group, and another belonging to the metal-poor group that dominates in the outer bulge. We detect the X-shape structure in fields with |Z| > 0.7 kpc and for stars with [Fe/H] > -0.5. Stars with [Fe/H] < -0.5 may form a spheroidal or "thick bar" distribution while those with [Fe/H] > -0.1 are strongly concentrated near the plane.

astro-ph.GA↗

arXiv2022

3D intrinsic shapes of quiescent galaxies in observations and simulations

We study the intrinsic 3D shapes of quiescent galaxies over the last half of cosmic history based on their axial ratio distribution. To this end, we construct a sample of unprecedented size, exploiting multi-wavelength $u$-to-$K_s$ photometry from the deep wide area surveys KiDS+VIKING paired with high-quality $i$-band imaging from HSC-SSP. Dependencies of the shapes on mass, redshift, photometric bulge prominence and environment are considered. For comparison, the intrinsic shapes of quenched galaxies in the IllustrisTNG simulations are analyzed and contrasted to their formation history. We find that over the full $0 10^{11}\ M_{\odot}$, with the effect being most pronounced at lower redshifts. In TNG, the most massive galaxies feature the highest ex-situ stellar mass fractions, pointing to violent relaxation via mergers as the mechanism responsible for their 3D shape transformation. Larger differences between observed and simulated shapes are found at low to intermediate masses. At any mass, the most spheroidal quiescent galaxies in TNG feature the highest bulge mass fractions, and conversely observed quiescent galaxies with the highest bulge-to-total ratios are found to be intrinsically the roundest. Finally, we detect an environmental influence on galaxy shape, at least at the highest masses, such that at fixed mass and redshift quiescent galaxies tend to be rounder in denser environments.

astro-ph.GA↗

arXiv2021

Gas Dynamics in the Galaxy: Total Mass Distribution and the Bar Pattern Speed

Gas morphology and kinematics in the Milky Way contain key information for understanding the formation and evolution of our Galaxy. We present a high resolution hydrodynamical simulation based on a realistic barred Milky Way potential constrained by recent observations. Our model can reproduce most features in the observed longitude-velocity diagram, including the Central Molecular Zone, the Near and Far 3-kpc arms, the Molecular Ring, and the spiral arm tangents. It can also explain the non-circular motions of masers obtained by the recent BeSSeL2 survey. The central gas kinematics are consistent with a mass of $6.9\times10^8\; {\rm M}_{\odot}$ in the Nuclear Stellar Disk. Our model predicts the formation of an elliptical gaseous ring surrounding the bar, which is composed of the 3-kpc arms, Norma arm, and the bar-spiral interfaces. This ring is similar to those "inner" rings in some Milky Way analogs with a boxy/peanut-shaped bulge. The kinematics of gas near the solar neighbourhood are governed by the Local arm, which is induced by the four major stellar spiral arms. The bar pattern speed constrained by our gas model is $37.5-40\; {\rm km}\;{\rm s}^{-1}\;{\rm kpc}^{-1}$, corresponding to a corotation radius of $R_{\rm CR}=6.0-6.4\;{\rm kpc}$. The rotation curve of our model rises gently within the central $\sim5\;{\rm kpc}$, which is significantly less steep than those predicted by modern zoom-in cosmological simulations such as Auriga.

astro-ph.GA↗

arXiv2021

Deprojection of external barred galaxies from photometry

The observations of external galaxies are projected to the 2D sky plane. Reconstructing the 3D intrinsic density distribution of a galaxy from the 2D image is challenging, especially for barred galaxies, but is a critical step for constructing galactic dynamical models. Here we present a method for deprojecting barred galaxies and we validate the method by testing against mock images created from an N-body simulation with a peanut-shaped bar. We decompose a galaxy image into a bulge (including a bar) and a disk. By subtracting the disk from the original image a barred bulge remains. We perform multi-Gaussian expansion (MGE) fit to each component, then we deproject them separately by considering the barred bulge is triaxial while the disk is axisymmetric. We restrict the barred bulge to be aligned in the disk plane and has a similar thickness to the disk in the outer regions. The 3D density distribution is thus constructed by combining the barred bulge and the disk. Our model can generally recover the 3D density distribution of disk and inner barred bulge regions, although not a perfect match to the peanut-shaped structure. By using the same initial conditions, we integrate the orbits in our model-inferred potential and the true potential by freezing the N-body simulation. We find that 85% of all these orbits have similar morphologies in these two potentials, and our model supports the orbits that generate a boxy/peanut-shaped structure and an elongated bar similar to these in the true potential.

astro-ph.GA↗

arXiv2021

The Flattening Metallicity Gradient in the Milky Way's Thin Disk

We calculate the ages, orbits and phase-space coordinates for a sample of $\sim$4 million LAMOST and Gaia stars. The ages are crossmatched and compared with values from two other popular age catalogs which derive the ages using different methods. Of these $\sim$4 million stars, we select a sample of 1.3 million stars and investigate their radial metallicity gradients (as determined by orbital radii) as a function of their ages. This analysis is performed on various subsets of the data split by chemistry and orbital parameters. We find that commonly used selections for ``thin disk'' stars (such as low-$α$ chemistry or vertically thin orbits) yield radial metallicity gradients which generally grow shallower for the oldest stars. We interpret this as a hallmark feature of radial migration (churning). Constraining our sample to very small orbital Z$_{max}$ (the maximal height of a star's integrated orbit) makes this trend most pronounced. A chemistry-based ``thin disk'' selection of $α$-poor stars displays the same trend, but to a lesser extent. Intruigingly, we find that ``thick disk'' selections in chemistry and Z$_{max}$ reveal a slightly positive radial metallicity gradients which seem similar in magnitude at all ages. This may imply that the thick disk population is well mixed in age, but not in radius. This finding could help constrain conditions during the early epochs of Milky Way formation, and shed light on processes such as the accretion and reaccretion of gasses of different metallicities.

astro-ph.GA↗

arXiv2021

Split invariant curves in rotating bar potentials

Invariant curves are generally closed curves in the Poincare's surface of section. Here we study an interesting dynamical phenomenon, first discovered by Binney et al. (1985) in a rotating Kepler potential, where an invariant curve of the surface of section can split into two disconnected line segments under certain conditions, which is distinctively different from the islands of resonant orbits. We first demonstrate the existence of split invariant curves in the Freeman bar model, where all orbits can be described analytically. We find that the split phenomenon occurs when orbits are nearly tangent to the minor/major axis of the bar potential. Moreover, the split phenomenon seems necessary to avoid invariant curves intersecting with each other. Such a phenomenon appears only in rotating potentials, and we demonstrate its universal existence in other general rotating bar potentials. It also implies that actions are no longer proportional to the area bounded by an invariant curve if the split occurs, but they can still be computed by other means.

nlin.CD↗

arXiv2021

An Empirical Proxy for the Second Integral of Motion in Rotating Barred or Tri-axial Potentials

We identify an effective proxy for the analytically-unknown second integral of motion (I_2) for rotating barred or tri-axial potentials. Planar orbits of a given energy follow a tight sequence in the space of the time-averaged angular momentum and its amplitude of fluctuation. The sequence monotonically traces the main orbital families in the Poincare map, even in the presence of resonant and chaotic orbits. This behavior allows us to define the "Calibrated Angular Momentum," the average angular momentum normalized by the amplitude of its fluctuation, as a numerical proxy for I_2. It also implies that the amplitude of fluctuation in L_z, previously under-appreciated, contains valuable information. This new proxy allows one to classify orbital families easily and accurately, even for real orbits in N-body simulations of barred galaxies. It is a good diagnostic tool of dynamical systems, and may facilitate the construction of equilibrium models.

astro-ph.GA↗