Identifying resonances of the Galactic bar in Gaia DR2: I. Clues from action space
Wilma H. Trick, Francesca Fragkoudi, Jason A. S. Hunt, J. Ted Mackereth, Simon D. M. White
备注：
17 pages, 12 figures, 1 table (excluding appendices); accepted for publication in MNRAS; this replacement contains minor fixes to match published version
发表信息：
Monthly Notices of the Royal Astronomical Society, Volume 500, Issue 2, January 2021, Pages 2645-2665
Action space synthesizes the orbital information of stars and is well-suited to analyse the rich kinematic substructure of the disc in the \emph{Gaia} DR2 radial velocity sample (RVS). We revisit the strong perturbation induced in the Milky Way (MW) disc by an $m=2$ bar, using test particle simulations and the actions $(J_R,L_z,J_z)$ estimated in an axisymmetric potential. These make three useful diagnostics cleanly visible. (1.) We use the well-known characteristic flip from outward to inward motion at the Outer Lindblad Resonance (OLR, $l=+1,m=2$), which occurs along the axisymmetric resonance line (ARL) in $(L_z,J_R)$, to identify in the \emph{Gaia} action data three candidates for the bar's OLR and pattern speed $Ω_\text{bar}$: $1.85Ω_0$, $1.20Ω_0$, and $1.63Ω_0$ (with $\sim0.1Ω_0$ systematic uncertainty). The \emph{Gaia} data is therefore consistent with both slow and fast bar models in the literature, but disagrees with recent measurements of $\sim1.45Ω_0$. (2.) For the first time, we demonstrate that bar resonances -- especially the OLR -- cause a gradient in vertical action $\langle J_z \rangle$ with $L_z$ around the ARL via "$J_z$-sorting" of stars. This could contribute to the observed coupling of $\langle v_R \rangle$ and $\langle | v_z | \rangle$ in the Galactic disc. (3.) We confirm prior results that the behaviour of resonant orbits is well approximated by scattering and oscillation in $(L_z,J_R)$ along a slope $ΔJ_R/ΔL_z = l/m$ centered on the $l$:$m$ ARL. Overall, we demonstrate that axisymmetrically estimated actions are a powerful diagnostic tool even in non-axisymmetric systems.
We investigate the nature of the double color-magnitude sequence observed in the Gaia DR2 HR diagram of stars with high transverse velocities. The stars in the reddest-color sequence are likely dominated by the dynamically-hot tail of the thick disk population. Information from Nissen & Schuster (2010) and from the APOGEE survey suggests that stars in the blue-color sequence have elemental abundance patterns that can be explained by this population having a relatively low star-formation efficiency during its formation. In dynamical and orbital spaces, such as the `Toomre diagram', the two sequ…
The Milky Way disk exhibits intricate orbit substructure of still-debated dynamical origin. The angle variables $(θ_ϕ,θ_R)$ -- which are conjugates to the actions $(L_z,J_R)$, and describe a star's location along its orbit -- are a powerful diagnostic to identify $l$:$m$ resonances via the orbit shape relation $Δθ_R / Δθ_ϕ= -m/l$. In the past, angle signatures have been hidden by survey selection effects (SEs). Using test particle simulations of a barred galaxy, we demonstrate that \emph{Gaia} should allow us to identify the Galactic bar's Outer Lindblad Resonance ($l=+1, m=2$, OLR) in angle s…
Our location in the Milky Way provides an exceptional opportunity to gain insight on the galactic evolution processes, and complement the information inferred from observations of external galaxies. Since the Milky Way is a barred galaxy, the study of motions of individual stars in the bulge and disc is useful to understand the role of the bar. The Gaia mission enables such study by providing the most precise parallaxes and proper motions to date. In this theoretical work, we explore the effects of the bar on the distribution of higher-order moments --the skewness and kurtosis-- of the proper …
In this first paper we simulate the population of disc Red Clump stars to be observed by Gaia. We generate a set of test particles and we evolve it in a 3D barred Milky Way like galactic potential. We assign physical properties of the Red Clump trace population and a realistic 3D interstellar extinction model. We add Gaia observational constraints and an error model according to the pre-commissioning scientific performance assessments. We present and analyse two mock catalogues, offered to the community, that are an excellent test bed for testing tools being developed for the future scientific…
From Gaia to GaiaNIR: II. A new view of the Milky Way bar
Lund University（隆德大学）；Universitat de Barcelona (UB)（巴塞罗那大学）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚空间研究学院）；INAF – Osservatorio Astrofisico di Torino（意大利国家天体物理研究所都灵天文台）；Observatoire Astronomique de Strasbourg, Université de Strasbourg/CNRS（斯特拉斯堡天文台，斯特拉斯堡大学/法国国家科学研究中心）；University of Surrey（萨里大学）
Using the VLBA, the BeSSeL survey has provided distances and proper motions of young massive stars, allowing an accurate measure of the Galactic spiral structure. By the same technique, we are planning to map the inner Galaxy using positions and velocities of evolved stars (provided by the BAaDE survey). These radio astrometric measurements (BeSSeL and BAaDE) will be complementary to Gaia results and the overlap will provide important clues on the intrinsic properties and population distribution of the stars in the bulge.
We discuss the origin of the LMC stellar bar by comparing the star formation histories (SFH) obtained from deep color-magnitude diagrams (CMDs) in the bar and in a number of fields in different directions within the inner disk. The CMDs, reaching the oldest main sequence turnoffs in these very crowded fields, have been obtained with VIMOS on the VLT in service mode, under very good seeing conditions. We show that the SFHs of all fields share the same patterns, with consistent variations of the star formation rate as a function of time in all of them. We therefore conclude that no specific even…
We analyse the kinematics of the Galactic bar-bulge using proper motions from the ESO public survey Vista Variables in the Via Lactea (VVV) and the second Gaia data release. Gaia has provided some of the first absolute proper motions within the bulge and the near-infrared VVV multi-epoch catalogue complements Gaia in highly-extincted low-latitude regions. We discuss the relative-to-absolute calibration of the VVV proper motions using Gaia. Along lines of sight spanning $-10<\ell/\,\mathrm{deg}<10$ and $-10<b/\,\mathrm{deg}<5$, we probabilistically model the density and velocity distributions a…
We analyze the kinematics and spatial distribution of 15,599 fundamental-mode RR Lyrae (RRL) stars in the Milky Way bulge by combining OGLE-IV photometric data and Gaia DR2 proper motions. We show that the longitudinal proper motions and the line-of-sight velocities can give similar results for the rotation in the Galactic central regions. The angular velocity of bulge RRLs is found to be around $35$ km s$^{-1}$ kpc$^{-1}$, significantly smaller than that for the majority of bulge stars ($50-60$ km s$^{-1}$ kpc$^{-1}$); bulge RRLs have larger velocity dispersion (120$-$140 km s$^{-1}$) than yo…
The length and pattern speed of the Milky Way bar are still controversial. Photometric and spectroscopic surveys of the inner Galaxy, as well as gas kinematics, favour a long and slowly rotating bar, with corotation around a Galactocentric radius of 6 kpc. On the other hand, the existence of the Hercules stream in local velocity space favours a short and fast bar with corotation around 4 kpc. This follows from the fact that the Hercules stream looks like a typical signature of the outer Lindblad resonance of the bar. As we showed recently, reconciling this local stream with a slow bar would ne…
Using a numerical simulation of an isolated barred disc galaxy, we first demonstrate that the resonances of the inner bar structure induce more prominent features in the action space distribution for the kinematically hotter stars, which are less sensitive to the local perturbation, such as the transient spiral arms. Then, we analyse the action distribution for the kinematically hotter stars selected from the Gaia EDR3 data as the stars with higher values of radial and vertical actions. We find several resonance features, including two new features, in the angular momentum distribution similar…
We have used Spitzer images of a sample of 68 barred spiral galaxies in the local universe to make systematic measurements of bar length and bar strength. We combine these with precise determinations of the corotation radii associated with the bars, taken from our previous study which used the phase change from radial inflow to radial outflow of gas at corotation, based on high resolution two-dimensional velocity fields in Hα taken with a Fabry-Pérot spectrometer. After presenting the histograms of the derived bar parameters, we study their dependence on the galaxy morphological type and on th…
The second data release of the Gaia mission has revealed, in stellar velocity and action space, multiple ridges, the exact origin of which is still debated. Recently, we demonstrated that a large Galactic bar with pattern speed 39 km/s/kpc does create most of the observed ridges. Among those ridges, the Hercules moving group would then be associated to orbits trapped at the co-rotation resonance of the bar. Here we show that a distinctive prediction of such a model is that the angular momentum of Hercules at the Sun's radius must significantly decrease with increasing Galactocentric azimuth, i…
We determine the mass of the Milky Way bar and the torque it causes, using Gaia DR2, by applying the orbital arc method. Based on this, we have found that the gravitational acceleration is not directed towards the centre of our Galaxy but a few degrees away from it. We propose that the tangential acceleration component is caused by the bar of the Galaxy. Calculations based on our model suggest that the torque experienced by the region around the Sun is $\approx 2400\, km^2 s^{-2}$ per solar mass. The mass estimate for the bar is $\sim 1.6\pm0.3\times10^{10} M_\odot$. Using greatly improved dat…
In this paper we introduce a new method for analysing Milky Way phase-space which allows us to reveal the imprint left by the Milky Way bar and spiral arms on the stars with full phase-space data in Gaia Data Release 2. The unprecedented quality and extended spatial coverage of these data enable us to discover six prominent stellar density structures in the disc to a distance of 5 kpc from the Sun. Four of these structures correspond to the spiral arms detected previously in the gas and young stars (Scutum-Centaurus, Sagittarius, Local and Perseus). The remaining two are associated with the ma…
We revisit the stellar velocity distribution in the Galactic bulge/bar region with APOGEE DR16 and {\it Gaia} DR2, focusing in particular on the possible high-velocity (HV) peaks and their physical origin. We fit the velocity distributions with two different models, namely with Gauss-Hermite polynomial and Gaussian mixture model (GMM). The result of the fit using Gauss-Hermite polynomials reveals a positive correlation between the mean velocity ($\bar{V}$) and the "skewness" ($h_{3}$) of the velocity distribution, possibly caused by the Galactic bar. The $n=2$ GMM fitting reveals a symmetric l…
We have developed our original made-to-measure (M2M) algorithm, PRIMAL, with the aim of modelling the Galactic disc from upcoming Gaia data. From a Milky Way like N-body disc galaxy simulation, we have created mock Gaia data using M0III stars as tracers, taking into account extinction and the expected Gaia errors. In PRIMAL, observables calculated from the N-body model are compared with the target stars, at the position of the target stars. Using PRIMAL, the masses of the N-body model particles are changed to reproduce the target mock data, and the gravitational potential is automatically adju…
We compare distance resolved, absolute proper motions in the Milky Way bar/bulge region to a grid of made-to-measure dynamical models with well defined pattern speeds. The data are obtained by combining the relative VVV Infrared Astrometric Catalog v1 proper motions with the Gaia DR2 absolute reference frame. We undertake a comprehensive analysis of the various errors in our comparison, from both the data and the models, and allow for additional, unknown, contributions by using an outlier-tolerant likelihood function to evaluate the best fitting model. We quantify systematic effects such as th…
Bar Formation During a Gaia-Sausage-Enceladus-like Merger Event
Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交大汤川研究所）；Department of Astronomy, School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院天文学系）；State Key Laboratory of Dark Matter Physics, School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院暗物质物理国家重点实验室）；LIRA, Observatoire de Paris, Université PSL, CNRS（巴黎天文台PSL大学CNRS LIRA）
A model of the Galaxy with the outer ring R1R2 can explain the observed distribution of the radial, VR, and azimuthal, VT, velocity components along the Galactocentric distance, R, derived from the Gaia EDR3 data. We selected stars from the Gaia EDR3 catalogue with reliable parallaxes, proper motions and line-of-sight velocities lying near the Galactic plane, |z|<200 pc, and in the sector of the Galactocentic angles |theta|<15 degrees and calculated the median velocities VR and VT in small bins along the distance R. The distribution of observed velocities appears to have some specific features…
We have derived absolute proper motions of the entire Galactic bulge region from VIRAC and Gaia. We present these as both integrated on-sky maps and, after isolating standard candle red clump (RC) stars, as a function of distance using RC magnitude as a proxy. These data provide a new global, 3-dimensional view of the Milky Way barred bulge kinematics. We find a gradient in the mean longitudinal proper motion, $<μ_l^\star>$, between the different sides of the bar, which is sensitive to the bar pattern speed. The split RC has distinct proper motions and is colder than other stars at similar dis…
Our recent studies of axial-symmetry breaking in the nearby ($d <3 \,{\rm kpc}$) star counts are sensitive to the distortions of stellar orbits perpendicular and parallel to the orientation of the bar just within and beyond the outer Lindblad resonance (OLR) radius. Using the location of the sign flip in the left-right asymmetry in stars counts about the anticenter line to determine the OLR radius $R_{\rm OLR}$, and treating the bar as if it were a weakly non-axisymmetric effect, we use $R_{\rm OLR}$ and recent measurements of the Galactic rotation curve and the Sun-Galactic-center distance $R…
The Influence of the Bar on the Dynamics of Globular Clusters in the Central Region of the Milky Way. Frequency Analysis of Orbits According to Gaia EDR3 Data
Super metal-rich (SMR) stars in the solar neighborhood are thought to be born in the inner disk and came to present location by radial migration, which is most intense at the co-rotation resonance (CR) of the Galactic bar. In this work, we show evidence for the CR origin of SMR stars in LAMOST-Gaia by detecting six ridges and undulations in the phi versus Lz space coded by median VR, following a similar slope of -8 km/s kpc/deg. The slope is predicted by Monario et al.'s model for CR of a large and slow Galactic bar. For the first time, we show the variation of angular momentum with azimuths f…
We have used the galaxy modeling algorithm \texttt{GravPot16}, to explore the more probable orbital elements of a sample of 64 selected N-rich stars across the Milky Way. Using the newly measured proper motions from \texttt{Gaia} DR2 with existing line-of-sight velocities from APOGEE-2 survey and spectrophotometric distance estimations from the \texttt{StarHorse}. We adopted a set of high-resolution particle simulations evolved in the same steady-state Galactic potential model with a bar, in order to identify the groups of N-rich stars that have a high probability of belonging to the bulge/bar…
A comprehensive kinematic model of the LMC disk from star clusters and field stars using Gaia DR3: Tracing the disk characteristics, rotation, bar, and the outliers
Using a single N-body simulation ($N=0.14\times 10^9$) we explore the formation, evolution and spatial variation of the phase-space spirals similar to those recently discovered by Antoja et al. in the Milky Way disk, with Gaia DR2. For the first time in the literature, we use a self-consistent N-body simulation of an isolated Milky Way-type galaxy to show that the phase-space spirals develop naturally from vertical oscillations driven by the buckling of the stellar bar. We claim that the physical mechanism standing behind the observed incomplete phase-space mixing process can be internal and n…
We explore the origin of phase-space substructures revealed by the second Gaia data release in the disc of the Milky Way, such as the ridges in the $V_ϕ$-$r$ plane, the undulations in the $V_ϕ$-$r$-$V_r$ space and the streams in the $V_ϕ$-$V_r$ plane. We use a collisionless N-body simulation with co-spatial thin and thick discs, along with orbit integration, to study the orbital structure close to the Outer Lindblad Resonance (OLR) of the bar. We find that a prominent, long-lived ridge is formed in the $V_ϕ$-$r$ plane due to the OLR which translates to streams in the $V_ϕ$-$V_r$ plane and exam…
We investigate the inner regions of the Milky Way with a sample of unprecedented size and coverage thanks to APOGEE DR16 and Gaia EDR3 data. Our inner Galactic sample has more than 26,000 stars within $|X_{\rm Gal}| <5$ kpc, $|Y_{\rm Gal}| <3.5$ kpc, $|Z_{\rm Gal}| <1$ kpc, and we also make the analysis for a foreground-cleaned sub-sample of 8,000 stars more representative of the bulge-bar populations. The inner Galaxy shows a clear chemical discontinuity in key abundance ratios [$α$/Fe], [C/N], and [Mn/O], probing different enrichment timescales, which suggests a star formation gap (quenching…
We have adapted our made-to-measure (M2M) algorithm PRIMAL to use mock Milky Way like data constructed from an N-body barred galaxy with a boxy bulge in a known dark matter potential. We use M0 giant stars as tracers, with the expected error of the ESA space astrometry mission Gaia. We demonstrate the process of constructing mock Gaia data from an N-body model, including the conversion of a galactocentric Cartesian coordinate N-body model into equatorial coordinates and how to add error to it for a single stellar type. We then describe the modifications made to PRIMAL to work with observationa…
Dynamical Origins of Azimuthal Metallicity Variations in the Galactic Disk: Insights from Kinematic Ridges with Gaia
The University of Texas at Austin（德克萨斯大学奥斯汀分校）；University of Surrey（萨里大学）；Flatiron Institute（平顿研究所）；University of Toronto（多伦多大学）；Princeton University（普林斯顿大学）
By taking advantage of the superb measurements of position and velocity for an unprecedented large number of stars provided in Gaia DR2, we have generated the first maps of the rotation velocity, $V_{\rm rot}$, and vertical velocity, $V_{\rm z}$, distributions as a function of the Galactocentric radius, $R_{\rm gal}$, across a radial range of $5<R_{\rm gal}<12$~kpc. In the $R-V_{\rm rot}$ map, we have identified many diagonal ridge features, which are compared with the location of the spiral arms and the expected outer Lindblad resonance of the Galactic bar. We have detected also radial wave-l…
We employ Gaia DR2 proper motions for 151 Milky Way globular clusters from Vasiliev (2019) in tandem with distances and line-of-sight velocities to derive their kinematical properties. To assign clusters to the Milky Way thick disk, bulge, and halo we follow the approach of Posti et al. (2018) who distinguished among different Galactic stellar components using stars's orbits. In particular, we use the ratio $L_{z}/e$, the $Z$ projection of the angular momentum to the eccentricity, as population tracer, which we complement with chemical abundances extracted from the literature and Monte-Carlo s…
Context: The high quality of the Gaia mission data is allowing to study the internal kinematics of the Large Magellanic Cloud (LMC) in unprecedented detail, providing insights on the non-axisymmetric structure of its disc. Aims: To define and validate an improved selection strategy to distinguish the LMC stars from the Milky Way foreground. To check the possible biases that assumed parameters or sample contamination from the Milky Way can introduce in the analysis of the internal kinematics of the LMC using Gaia data. Methods: Our selection is based on a supervised Neural Network classifier us…
The origin of the Hercules stream, the most prominent velocity substructure in the Solar neighbour disc stars, is still under debate. Recent accurate measurements of position, velocity, and metallicity provided by Tycho Gaia Astrometric Solution (TGAS) and RAdial Velocity Experiments (RAVE) have revealed that the Hercules stream is most clearly seen in the metal-rich region ([Fe/H] > 0), while it is not clearly seen in lower metallicity region ([Fe/H] < -0.25). By using a large number of chemo-dynamical 2D test-particle simulations with a rotating bar and/or spiral arms, we find that the obser…
The separation of oxygen- and carbon-rich AGB sources is crucial for their accurate use as local and cosmological distance and age/metallicity indicators. We investigate the use of unsupervised learning algorithms for classifying the chemistry of long-period variables from Gaia DR3's BP/RP spectra. Even in the presence of significant interstellar dust, the spectra separate into two groups attributable to O-rich and C-rich sources. Given these classifications, we utilise a supervised approach to separate O-rich and C-rich sources without BP/RP spectra but instead given broadband optical and inf…
The Galactic disk exhibits complex chemical and dynamical substructure thought to be induced by the bar, spiral arms, and satellites. Here, we explore the chemical signatures of bar resonances in action and velocity space and characterize the differences between the signatures of corotation and higher-order resonances using test particle simulations. Thanks to recent surveys, we now have large datasets containing metallicities and kinematics of stars outside the solar neighborhood. We compare the simulations to the observational data from Gaia EDR3 and LAMOST DR5 and find weak evidence for a s…
The second data release from ESA's Gaia mission has revealed many ridge-like structures in the velocity distribution of the Milky Way. We show that these can arise naturally from winding transient spiral structure that is commonly seen in N-body simulations of disk galaxies. We construct test particle models of the winding spiral structure, and compare the resulting distribution of orbits with the observed two-dimensional velocity distribution in the extended solar neighbourhood and with the distribution of rotational velocities over 8 kpc along the Sun--Galactic-centre--Galactic anti-centre l…
Using the method that was developed in the first paper of this series, we measure the vertical gravitational potential of the Galactic disk from the time-varying structure of the phase-space spiral, using data from Gaia as well as supplementary radial velocity information from legacy spectroscopic surveys. For eleven independent data samples, we inferred gravitational potentials that were in good agreement, despite the data samples' varied and substantial selection effects. Using a model for the baryonic matter densities, we inferred a local halo dark matter density of $0.0085 \pm 0.0039$ M$_\…
Gaia DR2 has provided an unprecedented wealth of information about the kinematics of stars in the Solar neighbourhood, and has highlighted the degree of features in the Galactic disc. We confront the data with a range of bar and spiral models in both action-angle space, and the $R_{\mathrm{G}}-v_ϕ$ plane. We find that the phase mixing induced by transient spiral structure creates ridges and arches in the local kinematics which are consistent with the Gaia data. We are able to produce a qualitatively good match to the data when combined with a bar with a variety of pattern speeds, and show that…
