arXiv subjects

Jason L. Sanders

Publications and source records attributed to Jason L. Sanders.

At least 19 recordsLinked to original sources

arXiv2026

A Novel Approach to 3D Dust Mapping of the Central Molecular Zone

The 3D distribution of dust and gas in the Milky Way's Central Molecular Zone (CMZ) is key to understanding gas inflows toward the Galactic Centre (GC), the process of star formation in this extreme environment, and the propagation of energetic cosmic rays originating from Sgr A*. However, while recent efforts have combined datasets in a Bayesian framework to estimate the near/far positions of individual molecular clouds in the CMZ, conflicts between different methodologies still remain and we are still lacking a comprehensive, model-independent map of all of the gas and dust in the CMZ, which is critical to address key science questions. Here we develop a new methodology to infer the 3D dust distribution of the CMZ. The key idea of the method is to use \emph{stellar} proper motions to get probabilistic information about the unknown stellar distances through a model of the distribution of star positions and velocities of the nuclear stellar disc (NSD), co-spatial to the CMZ. Taking \emph{stellar} proper motions and extinctions as input, the latter adopted as a proxy of the dust column density, the method returns the 3D dust distribution. It is non parametric, makes no a-priori assumption on the dust distribution, and is fundamentally distinct and largely independent of all existing methods. We show that the method can robustly and effectively reconstruct the mock 3D CMZ structure by testing it on a range of mock dust distributions, both analytically generated and taken from hydrodynamical simulations. Finally, we discuss the prospects for applying the method to real data.

astro-ph.GA↗

arXiv2026

Kinematic hints of a nuclear bar in the Milky Way

The Milky Way hosts a flattened nuclear stellar disc (NSD) that dominates the gravitational potential in the inner few hundred parsecs. Whether the NSD is purely axisymmetric or contains a nuclear bar remains an open question. We test for the presence of a nuclear bar using kinematic diagnostics by combining line-of-sight velocities from the KMOS NSD survey with proper motions from VIRAC2 to construct the $ (v_\ell, v_\mathrm{los}) $ velocity ellipse. After applying strict quality cuts to minimise contamination from large-scale bar stars, we measure the vertex deviation $ l_v $ and anisotropy $ \beta $ for several subsamples. For our primary sample ($ |\ell| < 0.9^\circ $, $ -0.4^\circ < b < 0.25^\circ $, $ \mathrm{[Fe/H]} > -0.3 $), we find a significant negative vertex deviation $ l_v = -54.8^{+13.1}_{-14.8}\,^\circ $ with moderate anisotropy $ \beta = 0.16^{+0.08}_{-0.05} $. A subsample restricted to the innermost four fields yields an even stronger signal with $ l_v = -64.3^{+12.1}_{-12.2}\,^\circ $ and $ \beta = 0.38^{+0.12}_{-0.07} $. The direction of maximum velocity dispersion is oriented along Galactic longitude, opposite to that observed in large-scale bar-dominated samples. These signatures are robust against extinction-driven incompleteness, primary-bar contamination, and the choice of metallicity threshold. They are inconsistent with an axisymmetric NSD or one oriented orthogonally to the primary bar, but match expectations for a nuclear bar oriented at $ \alpha \approx 60^\circ $-$75^\circ$ to the Sun-Galactic-Centre line with its near side pointing toward positive Galactic longitude. While definitive confirmation awaits larger and more precise samples from upcoming surveys, our results provide the first kinematic indication of a possible nuclear bar in the Milky Way.

astro-ph.GA↗

arXiv2026

Kinematic diagnostics for non-axisymmetry in the Milky Way's nuclear stellar disc

There is now strong evidence that the Milky Way (MW) hosts a nuclear stellar disc (NSD). However, whether the NSD is purely axisymmetric or contains a nuclear bar remains unresolved. Since approximately $50\%$ of barred galaxies with MW-like mass in the local Universe host a nuclear bar, investigating whether the MW hosts one is of interest. We conduct a systematic analysis to identify robust kinematic diagnostics capable of determining whether the MW hosts a nuclear bar. Using N-body simulations, we explore the kinematic signatures indicative of a nuclear bar. Using the phase-space coordinates longitude $(\ell)$, latitude $(b)$, proper motions ($\mu_\ell$ and $\mu_{\rm b})$ and line-of-sight velocity $(v_{\rm los})$, we test various diagnostics assuming different nuclear bar orientations. We also evaluate how sample size, dust extinction and bar amplitude influence the efficacy of the diagnostics. We identify two independent kinematic diagnostics capable of revealing a nuclear bar in the MW: (1) the vertex deviation, $l_{\rm v}$, of the ($v_{\ell}-v_{\rm los}$) velocity ellipse; and (2) The asymmetry in the $\mu_{\ell}$ vs $\ell$ distribution. While both are impacted by the sample size and extinction, the vertex deviation proves more robust, especially when combining stars from multiple observational fields. We also assess the correlation between the line-of-sight velocity and the $h_3$ Gauss-Hermite moment ("skewness") of the line-of-sight velocity but find no clear distinction between an NSD and a nuclear bar based on this metric. Our results suggest that data from the current KMOS survey may allow a marginal detection of a nuclear bar using the vertex deviation method. A companion paper provides further validation and detailed analysis of this approach. Nonetheless, future surveys will provide the high quality data necessary to fully exploit the diagnostics outlined in this study.

astro-ph.GA↗

arXiv2026

GSE vs. LMC: reshaping of radially biased stellar haloes by satellites

Perturbations from the Large Magellanic Cloud (LMC) of the Milky Way's stellar and dark matter haloes are well-established. However, studies have generally not considered the high radial anisotropy of the Milky Way's inner halo caused by the accreted debris of Gaia Sausage-Enceladus (GSE). We run a series of test particle simulations of stellar haloes being perturbed by the LMC, with different halo velocity anisotropies $\beta\in[0.5,0.9]$. The LMC causes these initially axisymmetric haloes to become approximately triaxial. Their major axes are aligned with its orbital plane and tilted by up to $\sim14^\circ$ with respect to a fixed Galactic disc. These effects become much more dramatic as $\beta$ increases, causing the halo to fractionate spatially according to anisotropy. This confirms the expectations of an analytical model, which predicts that orbits with eccentricities $e\gtrsim0.95$ should azimuthally align with the tidal field of the LMC. The reshaping of the $\beta=0.9$ halo creates strong overdensities of $\sim40\%$ at heliocentric distances as close as 15 kpc. These coincide with the well-known Virgo Overdensity (VOD) and Hercules-Aquila Cloud (HAC), which have previously been associated with the GSE. We propose that the HAC and VOD were created by the dynamical alignment of highly eccentric orbits by the LMC, and are not necessarily relics of the GSE merger geometry. We conclude that previous works have significantly underestimated perturbations from the LMC in the inner stellar halo by not considering sufficiently high velocity anisotropy. This effect should be corrected for when constructing equilibrium models of the GSE debris.

astro-ph.GA↗

arXiv2026

Quantifying the Milky Way, LMC and their interaction using all-sky kinematics of outer halo stars

The recent pericentric passage of the Large Magellanic Cloud (LMC) has dislodged the Milky Way's (MW) centre of mass, inducing dynamical disequilibrium, the reflex motion, in the kinematics of outer stellar halo stars. Using data out to $160 \, \rm kpc$ from the combined H3+SEGUE+MagE outer halo survey, we constrain the mass of the MW and LMC, as well as the resulting reflex motion and the stellar halo velocity anisotropy. Using a suite of 32,000 rigid MW--LMC simulations, each with a MW stellar halo evolved to the present day in the combined MW--LMC potential, we perform Simulation Based Inference by training a neural posterior estimator on the means and dispersions of the radial and tangential velocities of stars from the combined H3+SEGUE+MagE outer halo sample. Relative to halo stars at $100 \, \rm kpc$, we find the magnitude of the reflex velocity to be $v_{\rm travel} = 38.6^{+8.3}_{-7.8}\,\rm km \, s^{-1}$. Simultaneously, we determine the enclosed MW mass, $M_{\rm MW}(< 50 \, \rm kpc) = 3.36 \pm 0.15 \times 10^{11}\, \rm M_{\odot}$ and the enclosed LMC mass, $M_{\rm LMC}(< 50 \, \rm kpc) = 8.76^{+1.94}_{-1.77} \times 10^{10}\, \rm M_{\odot}$. Our results suggest that the total LMC mass must be at least $\sim20\%$ that of the MW. The velocity anisotropy prior to the LMC's infall is constrained to be $\beta_0 = 0.68 \pm 0.02$. Finally, we demonstrate that neglecting the LMC in models biases the estimated MW mass to prefer more massive values.

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arXiv2025

Transformational astrophysics and exoplanet science with Habitable Worlds Observatory's High Resolution Imager

Habitable Worlds Observatory (HWO) will be NASA's flagship space telescope of the 2040s, designed to search for life on other planets and to transform broad areas of astrophysics. NASA are seeking international partners, and the UK is well-placed to lead the design and construction of its imaging camera - which is likely to produce the mission's most visible public impact. Early participation in the mission would return investment to UK industry, and bring generational leadership for the UK in space science, space technology, and astrophysics.

astro-ph.IM↗

arXiv2025

Orbital migration and heating history of the Galactic disc: a transition between the bimodal discs

Stellar orbits in the Galactic disc evolve from their birth to the current shape through both radial migration and dynamical heating. The history of their secular evolution is imprinted in the current kinematics and age-metallicity distribution. We construct a chrono-chemo-dynamical model of the disc, incorporating inside-out growth, metallicity evolution, radial migration, and heating to fit the observed age-metallicity-kinematics distribution of LAMOST subgiant stars in both the low and high-$\alpha$ disc. By modelling all distribution parameters with spline fitting, we present the first non-parametric stellar migration and heating history of the Galaxy. We determine the heating-to-migration ratio, the ratio of the root-mean-square changes in radial/vertical and azimuthal actions, to be $\approx0.075$ for radial to azimuthal actions and $\approx0.015$ for vertical to azimuthal actions, implying a highly anisotropic diffusion in the action space. Furthermore, we identify a transition in radial migration efficiency coinciding with the transition moment of the bimodal disc, for which the radial migration was more efficient for the high-$\alpha$ disc than for the low-$\alpha$ disc. This transition may be attributed to two correlated scenarios: 1) a bar formation epoch accompanied by violent outward migration, and 2) a drop in the gas mass fraction in the disc when the low-$\alpha$ disc began to form. These findings offer further constraints on the formation mechanisms of bimodal discs, favouring the downsizing scenario. We also briefly discuss the connection between our results and recent high-redshift observations. In addition to the secular evolution history, our model maps the Milky Way ISM metallicity gradient at different lookback times, which we find has only varied a little (in the range of $-0.07~\rm to~-0.10~dex/kpc$) since disc formation.

astro-ph.GA↗

arXiv2025

Enhanced rates of stellar radial migration in gas-rich discs at high redshift

Radial migration and dynamical heating redistribute stars within galactic discs and thereby modify the chemo-kinematic structure of their host galaxies. Usually, these secular processes are studied in N-body and hydrodynamical simulations of Milky Way analogues with stellar-dominated discs. In contrast, discs at high redshift are gas-rich, which may qualitatively change how secular evolution proceeds. We use the Nexus framework to construct and evolve a suite of isolated galaxies with fixed halo and disc mass but varying initial disc gas fraction, from 0% to 100%. We show that in gas-rich models, the root-mean-square change in stellar angular momentum is up to a factor of two larger than in gas-poor analogues and is accompanied by stronger radial and vertical heating, leading to enhanced radial mixing. We further dissect the role of gas in specific migration channels. For bar-driven migration, corotation resonance dragging dominates in gas-poor discs, whereas in gas-rich discs, stars more readily reach and accumulate at the outer Lindblad resonance, which acts as a barrier. The high radial mixing efficiency in gas-rich phases can flatten the stellar metallicity gradient relative to that of the initial gaseous disc within only a few orbital timescales. Together, these results imply that radial mixing in early, gas-rich discs is substantially more vigorous than in late-time, gas-poor discs, naturally producing distinct evolutionary tracks for chemically bimodal discs such as that of the Milky Way.

astro-ph.GA↗

arXiv2025

Chemical separation of stellar populations: analytic solutions for chemical evolution models with metallicity-dependent yields

Stellar abundances of elements with production channels that are metallicity-dependent (most notably aluminium) have provided an empirical route for separating different Galactic components. We present 'single-zone' analytic solutions for the chemical evolution of galaxies when the stellar yields are metallicity-dependent. Our solutions assume a constant star formation efficiency, a constant mass-loading factor and that the yields are linearly dependent on the interstellar medium abundance (with the option of a saturation of the yields at high metallicity). We demonstrate how the metallicity dependence of the yields can be mathematically considered as a system-dependent delay time (approximately equal to the system's depletion time) that, when combined with system-independent delay times arising from stellar evolutionary channels, produces the separation of different systems based on their star formation efficiency and mass-loading factor. The utility of the models is highlighted through comparisons with data from the APOGEE spectroscopic survey. We provide a comprehensive discussion of the chemical evolution models in the [Al/Fe]-[Mg/Fe] plane, a diagnostic plane for the separation of in-situ and accreted Galactic components. Extensions of the models are presented, allowing for the modelling of more complex behaviours largely through the linear combination of the presented simpler solutions.

astro-ph.GA↗

arXiv2025

The Milky Way - Large Magellanic Cloud Interaction with Simulation Based Inference

The infall of the Large Magellanic Cloud (LMC) into the Milky Way (MW) has displaced the MW's centre of mass, manifesting as an observed reflex motion in the velocities of outer halo stars. We use a Simulation Based Inference framework to constrain properties of the MW, LMC and the induced reflex motion using the dynamics of outer MW halo stars. Specifically, we use the mean radial and tangential velocities of outer halo stars calculated in a set of distance and on-sky bins. We train neural networks to estimate parameter posterior distributions using a set of $128,000$ rigid MW--LMC simulations conditioned upon velocity data from the Dark Energy Spectroscopic Instrument (DESI) and the combined H3+SEGUE+MagE outer halo surveys. We constrain the reflex motion velocity and the enclosed LMC mass within $50 \, \rm kpc$ using the DESI or H3+SEGUE+MagE dataset while varying the survey sky coverage and depth. Using the radial and tangential velocity data from the H3+SEGUE+MagE survey and on-sky quadrants, we report a distance-averaged reflex motion velocity for the outer halo samples, the speed at which the MW lurches towards the LMC, of $v_{\rm{travel}} = 26.4^{+5.5}_{-4.4} \, \rm km \, \rm s^{-1}$, while simultaneously finding an enclosed LMC mass of $M_{\rm LMC}(< 50 \, \rm kpc) = 9.2^{+1.9}_{-2.3} \times 10^{10}\, \rm M_{\odot}$. Quoted uncertainties are statistical. Our results suggest that the LMC's total mass is at least $\approx 10-15 \%$ of that of the MW. This inference framework is flexible such that it can provide rapid constraints when applied to any future survey measuring the velocities of outer halo stars.

astro-ph.GA↗

arXiv2025

Geometry of the Milky Way's dark matter from dynamical models of the tilted stellar halo

The shape and orientation of the Milky Way's dark matter halo remain poorly constrained. Observations of the accreted stellar halo show that it is triaxial and tilted with respect to the disc. If this configuration is long-lived, it can place constraints on the shape and orientation of the dark matter halo that can support it close to steady state. We use a novel method to fit equilibrium orbit-superposition (Schwarzschild) models to the stellar halo in a realistic Milky Way potential with a tilted dark matter halo. We assume that the long axes of each halo and the disc normal are coplanar. These models are matched to parametric density fits and velocity anisotropy measurements of Gaia Sausage-Enceladus (GSE) stars at radii $r\in[6,60]$ kpc. The observations are consistent with a (near-)prolate dark matter halo whose density has a short-to-long axis ratio of $q_\mathrm{dm}=0.87_{-0.09}^{+0.05}$. The long axis is inclined at an angle of $\beta_\mathrm{dm}=43_{-8}^{+22}\,^\circ$ to the disc plane, which exceeds the stellar halo tilt by $\approx18^\circ$. Spherical haloes cannot support the observed structure of the GSE in equilibrium. The best-fitting dynamical GSE model has a radius-dependent shape and orientation; between radii of 6 and 60~kpc the tilt increases from $\beta_*(r)\approx10^\circ$ to $\approx35^\circ$. Our model provides a good fit to the observed triaxial structure and dynamics of the GSE. It is therefore an excellent source of realistic initial conditions for simulations of the halo, such as for investigating perturbations from satellites or the Galactic bar.

astro-ph.GA↗

arXiv2025

Carbon-Enhanced Dwarf Stars Are Predominantly a Halo Population

This paper reports a Galactic kinematical and dynamical analysis of 1003 main-sequence carbon stars. The sample is drawn from the Sloan Digital Sky Survey, and cross-matched with Gaia DR3 to obtain 6-dimensional positions and velocities using a Bayesian framework. The study provides the first reliable distances for a large sample of dwarf carbon stars, which are then analyzed using both space motions and actions. The results are combined with dynamical equilibrium models for the three primary Galactic components to assign membership, finding that around 60 per cent belong to the halo, and over 30 per cent originate in the thick disc. Therefore, the results indicate dwarf carbon stars are dominated by a metal-poor halo population, and are thus an excellent resource for stellar archaeology. These stars remain on the main sequence and are relatively nearby, but atmospheric modelling is challenged by their cool effective temperatures and strong molecular features. In light of this, efforts should be made to improve C/O >1 atmospheric modelling, as the subset of low-mass dwarf carbon stars may numerically dominate the Galactic population of carbon-enriched, metal-poor stars.

astro-ph.SR↗

arXiv2025

Thick-to-thin disc transition and gas disc shrinking induced by the Gaia-Sausage-Enceladus merger

Understanding the Milky Way disc formation requires characterising its structural and kinematic properties as functions of stellar age. Using red giant stars from APOGEE DR17 and Gaia DR3, we model the age-dependent stellar kinematics with a quasi-isothermal distribution function and fit disc parameters as a function of age using non-parametric splines. We identify a transition from thick to thin disc populations around 10 Gyr ago. Stars older than this have short scale lengths ($\sim$1.7 kpc), typical of the thick disc, while younger stars exhibit increasing scale length with decreasing age, consistent with inside-out formation of the thin disc. This transition possibly coincides with the end of the starburst triggered by the Gaia-Sausage-Enceladus (GSE) merger. Stars formed around 10 Gyr ago exhibit a dip in scale length, even shorter than that of the thick disc. Comparison with an Auriga simulation suggests that this scale-length dip reflects gas disc shrinking caused by the transition from a cold to hot gas accretion mode. We propose the following disc formation scenario: (1) the thick disc formed under cold-mode accretion; (2) the GSE merger triggered a starburst and increased the total mass of the Galaxy, causing the transition to hot-mode accretion; (3) rapid gas consumption led to temporary shrinking of the star-forming gas disc; and then (4) thin disc grows in an inside-out fashion, as the size of the star-forming gas disc grows via hot-mode smooth gas accretion.

astro-ph.GA↗

arXiv2025

ChemZz I: Comparing Oxygen and Iron Abundance Patterns in the Milky Way, the Local Group and Cosmic Noon

Our understanding of the chemical evolution of galaxies has advanced through measurements from both distant galaxies across redshift, and our own Milky Way (MW). To form a comprehensive picture, it is essential to unify these constraints, placing them on a common scale and parlance and to understand their systematic differences. In this study, we homogenize oxygen and iron measurements from star-forming galaxies at Cosmic Noon ($z{\sim}2-3$) with resolved stellar abundances from the Local Group. The MW is divided into four components, assuming the outer halo is dominated by debris from the Gaia-Sausage-Enceladus (GSE) progenitor. After converting all abundances to a common Solar scale, we identify clear $\alpha$- and iron-enhancement trends with mass in the $z{\sim}2-3$ galaxies and find good agreement between these galaxies and the MW high-$\alpha$ disc in [O/Fe] vs. [Fe/H]. We also find excellent agreement between the [O/Fe] trends seen in the MW high- and low-$\alpha$ discs with O-abundances seen in old and young planetary nebulae in M~31 respectively, supporting the existence of $\alpha$-bimodality in the inner regions of M~31. Finally, we use globular cluster ages to project the MW and GSE back in time to $z{\sim}3$ and find that their estimated mass, oxygen and iron abundances are strikingly consistent with the mass-metallicity relation of star-forming galaxies at $z{\sim}3$. In the future, increased transparency around the choice of Solar scale and abundance methodology will make combining chemical abundances easier -- contributing to a complete picture of the chemical evolution of all galaxies.

astro-ph.GA↗

arXiv2025

A Simulation Based Inference Approach to the Dynamics of the MW-LMC System -- Validation

The infall of the LMC into the Milky Way (MW) has generated dynamical disequilibrium throughout the MW. The interaction has displaced the MW's centre of mass, manifesting as an apparent 'reflex motion' in velocities of outer halo stars. Often, expensive high fidelity MW--LMC simulations are required to model these effects, though the range of model parameter spaces can be large and complex. We investigate the ability of lower fidelity, rigid MW-LMC simulations to reliably infer the model parameters of higher fidelity N-body and hydrodynamical cosmological zoom-in MW--LMC simulations using a Simulation-Based Inference (SBI) approach. We produce and release a set of 128,000 MW--LMC rigid potentials, with stellar haloes evolved to present-day, each adopting a unique combination of model parameters including the MW mass, the LMC mass and the dynamical friction strength. For these simulation parameters, we use SBI to find their posterior distributions. We find that our SBI framework trained on rigid MW--LMC simulations is able to correctly infer the true simulation LMC mass within a $1\sigma$ confidence interval from both N-body and cosmological simulations when knowledge of the induced MW reflex motion is provided as data. This motivates future applications of the presented SBI framework to observational data, which will help constrain both MW and LMC properties, as well as the dynamics of the MW's reflex motion.

astro-ph.GA↗

arXiv2025

Bar-driven dispersal of Galactic substructure

Galactic archaeologists often assume that integrals of motion (IoMs) such as $L_z$ and $E$ are conserved, so substructure remains frozen in IoM space over many Gyr. However, this is not true in the Milky Way due in part to its rotating bar. In this study we quantify the effects of the bar on the dynamics of substructure. We employ three different theoretical models: an analytical toy model; a set of test particle simulations with steady and slowing bars; and a cosmological zoom-in simulation of a Milky Way-like galaxy. Each model predicts that the bar increases the angular momentum and energy spread of low-energy substructures by a factor of $\sim10-100$, so they cannot remain tightly clustered. We derive a criterion for determining when this effect is important. The most affected orbits are low energy ($E\lesssim E_\odot$, $r_\mathrm{apo}<40$ kpc), prograde, eccentric, or low inclination. This includes $\sim3/4$ of Galactic globular clusters and $\sim1/4$ of known stellar streams. We predict the presence of abundant bar-dispersed substructure. The structures remain much more tightly clustered in the space of metallicity and Jacobi integral $H_\mathrm{J}=E-\Omega_\mathrm{b}L_z$. We therefore propose using $H_\mathrm{J}$ and chemistry instead of traditional IoMs when searching for inner halo substructure. In $(L_z,E)$ space the dispersal of the structures is along a principal direction with gradient $\mathrm{d}E/\mathrm{d}L_z$ equal to the bar's pattern speed $\Omega_\mathrm{b}$. Bar-dispersed substructure should therefore allow the past evolution of $\Omega_\mathrm{b}$ to be constrained.

astro-ph.GA↗

arXiv2025

Denoising Milky Way stellar survey data with normalizing flow models

The Gaia dataset has revealed many intricate Milky Way substructures in exquisite detail, including moving groups and the phase spiral. Precise characterisation of these features and detailed comparisons to theoretical models require engaging with Gaia's heteroscedastic noise model, particularly in more distant parts of the Galactic disc and halo. We propose a general, novel machine-learning approach using normalizing flows for denoising density estimation, with particular focus on density estimation from stellar survey data such as that from Gaia. Normalizing flows transform a simple base distribution into a complex target distribution through bijective transformations resulting in a highly expressive and flexible model. The denoising is performed using importance sampling. We demonstrate that this general procedure works excellently on Gaia data by reconstructing detailed local velocity distributions artificially corrupted with noise. For example, we show the multiple branches of the Hercules stream and the phase-space spiral can both be well captured by our model. We discuss hyperparameter choice to optimally recover substructure and compare our approach to extreme deconvolution. The model therefore promises to be a robust tool for studying the Milky Way's kinematics in Galactic locations where the noise from Gaia is significant.

astro-ph.GA↗

arXiv2025

Dynamical streams in the local stellar halo

Co-moving groups of stars (streams) are well known in the velocity space of the disc near the Sun. Many are thought to arise from resonances with the Galactic bar or spiral arms. In this work, we search for similar moving groups in the velocity space of the halo, at low angular momentum. From the asymmetry of the radial velocity distribution $v_R$, we identify two inward-moving streams with $v_R<0$ and small $|v_\phi|$. These are projections of the `chevrons' previously discovered in radial phase space $(R,v_R)$. A test particle simulation in a realistic Milky Way potential with a decelerating bar naturally produces analogues of these features, and they are observed across a wide range of metallicity. They are therefore very likely to be dynamical streams created by trapping in the bar's resonances. Specifically, they occupy regions of phase space where orbits are trapped in the corotation and outer Lindblad resonances respectively. By tracing these streams across a range of radii in $(R,v_R)$ space, we fit resonant orbits to their tracks in a flexible potential with variable bar pattern speed. This allows us to simultaneously constrain the mass profile of the Milky Way for $r\lesssim20$ kpc and the pattern speed $\Omega_\mathrm{b}$. We estimate the mass enclosed within $r=20$ kpc to be $M_{20}=(2.17\pm0.21)\times10^{11}M_\odot$, and the pattern speed to be $\Omega_\mathrm{b}=31.9_{-1.9}^{+1.8}$ km/s/kpc. Our fitted potential is in excellent agreement with previous results, while we favour a slightly slower pattern speed than most recent estimates.

astro-ph.GA↗