分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Cosmological simulations show that dark matter halos surrounding baryonic disks have a wide range of angular momenta, measured by the spin parameter ($\lambda$). 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 $\lambda$ from 0 to 0.1 for co-rotating (prograde) spinning halos and one counter-rotating (retrograde) halo spin ($\lambda$=-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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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 ($\sigma_{z}$) is decreased relative to the other side of the Galactic plane. Fitting only to the observed asymmetric N/S $\sigma_{z}$ 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Junkai Zhang
Stijn Wuyts
Callum Witten
Charlotte R. Avery
Lei Hao
Raman Sharma
Juntai Shen
Jun Toshikawa
Carolin Villforth
摘要： 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.
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