scholarly journals Bar resonances and low angular momentum moving groups in the Galaxy revealed by their stellar ages

2020 ◽  
Vol 643 ◽  
pp. L3 ◽  
Author(s):  
Chervin F. P. Laporte ◽  
Benoit Famaey ◽  
Giacomo Monari ◽  
Vanessa Hill ◽  
Christopher Wegg ◽  
...  
Keyword(s):  
Phase Space ◽  
Milky Way ◽  
Azimuthal Velocity ◽  
Velocity Versus ◽  
Dynamical Structure ◽  
Angular Momenta ◽  
Stellar Ages ◽  
The Galaxy ◽  
Moving Groups ◽  
Hydrodynamical Simulations

We use the second Gaia data release to dissect the Milky Way disc in phase-space and relative ages. We confirm and report the existence of multiple velocity moving groups at low azimuthal velocities and angular momenta, below Arcturus, regularly separated by ∼18−20 km s−1 in azimuthal velocity. Such features were predicted to exist more than ten years ago, based on the reaction of the Milky Way to a perturbation in the disc undergoing phase-mixing. These structures appear slightly younger than their phase-space surroundings and reach up to high (solar) metallicities, which argues against an extra-galactic origin. We also identify, in terms of relative age, many of the classical ridges in the plane of azimuthal velocity versus Galactocentric radius, which are traditionally associated with resonance features. These ridges are also younger than their phase-space surroundings in accordance with predictions from recent state-of-the-art cosmological hydrodynamical simulations of Milky Way-like galaxies. We study the response of dynamically young and old stellar disc populations to resonances from an analytic model of a large bar which, remarkably, reproduces qualitatively the trends seen in the data for the classical ridges close to circularity. Our results reinforce the idea that the Galactic disc is being shaped by both internal and external perturbations, along with the fact that while absolute isochrone ages have to be taken with great care, exploring the dynamical structure of the disc in stellar ages, especially with future asteroseismic data, will provide much stronger constraints than metallicity and abundance trends alone.

scholarly journals Streams, Substructures, and the Early History of the Milky Way

2020 ◽  
Vol 58 (1) ◽  
pp. 205-256 ◽  
Author(s):  
Amina Helmi
Keyword(s):  
Star Formation ◽  
Phase Space ◽  
Milky Way ◽  
Current Knowledge ◽  
Thick Disk ◽  
Age Dating ◽  
Wide Field ◽  
Chemical Labeling ◽  
The Galaxy ◽  
Hydrodynamical Simulations

The advent of the second data release of the Gaia mission, in combination with data from large spectroscopic surveys, is revolutionizing our understanding of the Galaxy. Thanks to these transformational data sets and the knowledge accumulated thus far, a new, more mature picture of the evolution of the early Milky Way is currently emerging. ▪  Two of the traditional Galactic components, namely, the stellar halo and the thick disk, appear to be intimately linked: Stars with halo-like kinematics originate in similar proportions from a heated (thick) disk and from debris from a system named Gaia-Enceladus. Gaia-Enceladus was the last big merger event experienced by the Milky Way and was completed around 10 Gyr ago. The puffed-up stars now present in the halo as a consequence of the merger have thus exposed the existence of a disk component at z ∼ 1.8. This is likely related to the previously known metal-weak thick disk and may be traceable to metallicities [Fe/H] [Formula: see text] −4. As importantly, there is evidence that the merger with Gaia-Enceladus triggered star formation in the early Milky Way, plausibly leading to the appearance of the thick disk as we know it. ▪  Other merger events have been characterized better, and new ones have been uncovered. These include, for example, the Helmi streams, Sequoia, and Thamnos, which add to the list of those discovered in wide-field photometric surveys, such as the Sagittarius streams. Current knowledge of their progenitors’ properties, star formation, and chemical evolutionary histories is still incomplete. ▪  Debris from different objects shows different degrees of overlap in phase-space. This sometimes confusing situation can be improved by determining membership probabilities via quantitative statistical methods. A task for the next few years will be to use ongoing and planned spectroscopic surveys for chemical labeling and to disentangle events from one another using dimensions other than phase-space, metallicity, or [α/Fe]. ▪  These large surveys will also provide line-of-sight velocities missing for faint stars in Gaia releases and more accurate distance determinations for distant objects, which in combination with other surveys could also lead to more accurate age dating. The resulting samples of stars will cover a much wider volume of the Galaxy, allowing, for example, the linking of kinematic substructures found in the inner halo to spatial overdensities in the outer halo. ▪  All the results obtained so far are in line with the expectations of current cosmological models. Nonetheless, tailored hydrodynamical simulations to reproduce in detail the properties of the merger debris, as well as constrained cosmological simulations of the Milky Way, are needed. Such simulations will undoubtedly unravel more connections between the different Galactic components and their substructures, and will aid in pushing our knowledge of the assembly of the Milky Way to the earliest times.

scholarly journals Riding the kinematic waves in the Milky Way disk with Gaia

2018 ◽  
Vol 619 ◽  
pp. A72 ◽  
Author(s):  
P. Ramos ◽  
T. Antoja ◽  
F. Figueras
Keyword(s):  
Galactic Disk ◽  
Average Rate ◽  
Statistical Significance ◽  
Azimuthal Velocity ◽  
Spatial Evolution ◽  
Phase Mixing ◽  
The Galaxy ◽  
Dimensional Measurements ◽  
Moving Groups ◽  
New Phase

Context. Gaia DR2 has delivered full-sky six-dimensional measurements for millions of stars, and the quest to understand the dynamics of our Galaxy has entered a new phase. Aims. Our aim is to reveal and characterise the kinematic substructure of the different Galactic neighbourhoods, to form a picture of their spatial evolution that can be used to infer the Galactic potential, its evolution, and its components. Methods. We take approximately 5 million stars in the Galactic disk from the Gaia DR2 catalogue and build the velocity distribution in different Galactic neighbourhoods distributed along 5 kpc in Galactic radius and azimuth. We decompose their distribution of stars in the VR–Vϕ plane with the wavelet transformation and asses the statistical significance of the structures found. Results. We detect distinct kinematic substructures (arches and more rounded groups) that diminish their azimuthal velocity as a function of Galactic radius in a continuous way, connecting volumes up to 3 kpc apart in some cases. The average rate of decrease is ∼23 km s−1 kpc−1. In azimuth, the variations are much smaller. We also observe different behaviours: some approximately conserve their vertical angular momentum with radius (e.g. Hercules), while others seem to have nearly constant kinetic energy (e.g. Sirius). These two trends are consistent with the approximate predictions of resonances and phase mixing, respectively. Besides, the overall spatial evolution of Hercules is consistent with being related to the outer Lindblad resonance of the Galactic bar. In addition, we detect new kinematic structures that only appear at either inner or outer Galactic radius, different from the solar neighbourhood. Conclusions. The strong and distinct variation observed for each kinematic substructure with position in the Galaxy, along with the characterisation of extrasolar moving groups, will allow to better model the dynamical processes affecting the velocity distributions.

scholarly journals Reconstructing the Last Major Merger of the Milky Way with the H3 Survey

2021 ◽  
Vol 923 (1) ◽  
pp. 92
Author(s):  
Rohan P. Naidu ◽  
Charlie Conroy ◽  
Ana Bonaca ◽  
Dennis Zaritsky ◽  
Rainer Weinberger ◽  
...  
Keyword(s):  
Milky Way ◽  
Age Distribution ◽  
Major Axis ◽  
Angular Momenta ◽  
Outer Disk ◽  
The Galaxy ◽  
Stellar Halo ◽  
Major Merger ◽  
Triaxial Shape

Abstract Several lines of evidence suggest that the Milky Way underwent a major merger at z ∼ 2 with the Gaia-Sausage-Enceladus (GSE) galaxy. Here we use H3 Survey data to argue that GSE entered the Galaxy on a retrograde orbit based on a population of highly retrograde stars with chemistry similar to the largely radial GSE debris. We present the first tailored N-body simulations of the merger. From a grid of ≈500 simulations we find that a GSE with M ⋆ = 5 × 108 M ⊙, M DM = 2 × 1011 M ⊙ best matches the H3 data. This simulation shows that the retrograde stars are stripped from GSE’s outer disk early in the merger. Despite being selected purely on angular momenta and radial distributions, this simulation reproduces and explains the following phenomena: (i) the triaxial shape of the inner halo, whose major axis is at ≈35° to the plane and connects GSE’s apocenters; (ii) the Hercules-Aquila Cloud and the Virgo Overdensity, which arise due to apocenter pileup; and (iii) the 2 Gyr lag between the quenching of GSE and the truncation of the age distribution of the in situ halo, which tracks the lag between the first and final GSE pericenters. We make the following predictions: (i) the inner halo has a “double-break” density profile with breaks at both ≈15–18 kpc and 30 kpc, coincident with the GSE apocenters; and (ii) the outer halo has retrograde streams awaiting discovery at >30 kpc that contain ≈10% of GSE’s stars. The retrograde (radial) GSE debris originates from its outer (inner) disk—exploiting this trend, we reconstruct the stellar metallicity gradient of GSE (−0.04 ± 0.01 dex r 50 − 1 ). These simulations imply that GSE delivered ≈20% of the Milky Way’s present-day dark matter and ≈50% of its stellar halo.

NIHAO-UHD: High-resolution Simulations of MW mass galaxies

2017 ◽  
Vol 13 (S334) ◽  
pp. 209-212
Author(s):  
Tobias Buck ◽  
Andrea Macciò ◽  
Melissa Ness ◽  
Aura Obreja ◽  
Aaron Dutton
Keyword(s):  
High Resolution ◽  
Milky Way ◽  
Satellite System ◽  
The Galaxy ◽  
First Results ◽  
History Of ◽  
Galaxy Center ◽  
Hydrodynamical Simulations ◽  
Very High ◽  
Disc Galaxies

AbstractHigh resolution cosmological and hydrodynamical simulations have reached a resolution able to resolve in a self consistent way the disc of our galaxy, the galaxy center and the satellites orbiting around it. We present first results from the NIHAO-UHD project, a set of very high-resolution baryonic zoom-in simulations of Milky Way mass disc galaxies. These simulations model the full cosmological assembly history of the galaxies and their satellite system using the same, well tested physics as the NIHAO project. We show that these simulations can self-consistently reproduce the observed kinematical and morphological features of the X-shaped bulge observed in our own Milky Way.

scholarly journals The orbital evolution of UFDs and GCs in an evolving Galactic potential

2020 ◽  
Vol 500 (3) ◽  
pp. 2937-2957
Author(s):  
Benjamin M Armstrong ◽  
Kenji Bekki ◽  
Aaron D Ludlow
Keyword(s):  
Globular Clusters ◽  
Milky Way ◽  
Orbital Evolution ◽  
Orbital Parameters ◽  
Angular Momenta ◽  
Galactic Potential ◽  
The Galaxy ◽  
Galaxy Masses ◽  
The Difference ◽  
Evolving Models

ABSTRACT We use the second Gaia data release to investigate the kinematics of 17 ultra-faint dwarf galaxies (UFDs) and 154 globular clusters (GCs) in the Milky Way, focusing on the differences between static and evolving models of the Galactic potential. An evolving potential modifies a satellite’s orbit relative to its static equivalent, though the difference is small compared to existing uncertainties on orbital parameters. We find that the UFD Boötes II is likely on its first passage around the Milky Way. Depending on the assumed mass of the Milky Way, the UFDs Triangulum II, Hydrus I, Coma Berenices, Draco II, and Ursa Major II, as well as the GC Pyxis, may also be on first infall so may be useful for constraining the mass of the Galaxy. We identify a clear kinematic distinction between metal-rich ([Fe/H] > −1.1) and metal-poor GCs ([Fe/H] ≤ −1.1). Although most metal-rich clusters occupy predominately prograde orbits, with low eccentricities (e ≈ 0.35) and similar specific angular momenta and orbital planes as the Galactic disc, seven show potentially retrograde orbits, the origin of which is unclear. Metal-poor clusters have more diverse orbits, higher eccentricities (e ≈ 0.65), and half of them have orbital planes offset from the disc by 60° to 120°—twice as many as the metal-poor GCs. The UFDs have similar θ and ϕ to the metal-poor GCs, suggesting a similar origin. We provide a catalogue of orbital parameters for UFDs and GCs for two different Galaxy masses and their observational uncertainties.

scholarly journals The orbital phase space of contracted dark matter haloes

2020 ◽  
Vol 495 (1) ◽  
pp. 12-28 ◽  
Author(s):  
Thomas M Callingham ◽  
Marius Cautun ◽  
Alis J Deason ◽  
Carlos S Frenk ◽  
Robert J J Grand ◽  
...  
Keyword(s):  
Dark Matter ◽  
Phase Space ◽  
Total Mass ◽  
Milky Way ◽  
Direct Detection ◽  
Baryon Density ◽  
Orbital Phase ◽  
Mass Profile ◽  
Hydrodynamical Simulations ◽  
Radial Action

ABSTRACT We study the orbital phase space of dark matter (DM) haloes in the auriga suite of cosmological hydrodynamics simulations of Milky Way (MW) analogues. We characterize haloes by their spherical action distribution, $F\left(J_{{r}},L\right)$, a function of the specific angular momentum, L, and the radial action, Jr, of the DM particles. By comparing DM-only and hydrodynamical simulations of the same haloes, we investigate the contraction of DM haloes caused by the accumulation of baryons at the centre. We find a small systematic suppression of the radial action in the DM haloes of the hydrodynamical simulations, suggesting that the commonly used adiabatic contraction approximation can result in an underestimate of the density by $\sim 8{{ \rm {per\ cent}}}$. We apply an iterative algorithm to contract the auriga DM haloes given a baryon density profile and halo mass, recovering the true contracted DM profiles with an accuracy of $\sim 15{{ \rm {per\ cent}}}$, that reflects halo-to-halo variation. Using this algorithm, we infer the total mass profile of the MW’s contracted DM halo. We derive updated values for the key astrophysical inputs to DM direct detection experiments: the DM density and velocity distribution in the Solar neighbourhood.

scholarly journals The biggest splash

2020 ◽  
Vol 494 (3) ◽  
pp. 3880-3898 ◽  
Author(s):  
Vasily Belokurov ◽  
Jason L Sanders ◽  
Azadeh Fattahi ◽  
Martin C Smith ◽  
Alis J Deason ◽  
...  
Keyword(s):  
Milky Way ◽  
Large Population ◽  
Azimuthal Velocity ◽  
Causal Connection ◽  
Elemental Abundances ◽  
Eccentric Orbits ◽  
Stellar Ages ◽  
Major Merger ◽  
Nearby Stars ◽  
Rule Out

ABSTRACT Using a large sample of bright nearby stars with accurate Gaia Data Release 2 astrometry and auxiliary spectroscopy we map out the properties of the principle Galactic components such as the ‘thin’ and ‘thick’ discs and the halo. We confirm previous claims that in the Solar neighbourhood, there exists a large population of metal-rich ([Fe/H] > −0.7) stars on highly eccentric orbits. By studying the evolution of elemental abundances, kinematics, and stellar ages in the plane of azimuthal velocity vϕ and metallicity [Fe/H], we demonstrate that this metal-rich halo-like component, which we dub the Splash, is linked to the α-rich (or ‘thick’) disc. Splash stars have little to no angular momentum and many are on retrograde orbits. They are predominantly old, but not as old as the stars deposited into the Milky Way (MW) in the last major merger. We argue, in agreement with several recent studies, that the Splash stars may have been born in the MW’s protodisc prior to the massive ancient accretion event which drastically altered their orbits. We cannot, however, rule out other (alternative) formation channels. Taking advantage of the causal connection between the merger and the Splash, we put constraints of the epoch of the last massive accretion event to have finished 9.5 Gyr ago. The link between the local metal-rich and metal-poor retrograde stars is confirmed using a large suite of cutting-edge numerical simulations of the MW’s formation.

scholarly journals The Light Side and The Dark Side of the Milky Way Halo

2013 ◽  
Vol 9 (S298) ◽  
pp. 416-416
Author(s):  
Prajwal R. Kafle ◽  
Sanjib Sharma ◽  
Geraint F. Lewis ◽  
Joss Bland-Hawthorn
Keyword(s):  
Milky Way ◽  
Dynamical Property ◽  
Azimuthal Velocity ◽  
Dark Matter Halo ◽  
Dark Side ◽  
Recent Measurement ◽  
Velocity Anisotropy ◽  
The Galaxy ◽  
Stellar Halo ◽  
Light Side

AbstractWe present our recent measurement of the kinematics of the Milky Way stellar halo (Light Side) and the derived mass of the dark matter halo (Dark Side) using the Jeans analysis. A tangential dip in the velocity anisotropy profile at r ~ 17 kpc (Kafle et al. 2012), and a distinct difference of ~65 kms−1 in the mean azimuthal velocity and the r.m.s dispersion of the most metal-rich and the metal-poor Blue Horizontal Branch stars we find (Kafle et al. 2013) are reported. The implications of this on the current controversial issue of an existence of the two-components in the halo are also discussed.Aided with the kinematic measurements of the light side, we demonstrate how we infer the dynamical property of the dark side. Considering a realistic three component galaxy model (Hernquist bulge, Miyamoto-Nagai disk and NFW halo), we estimate the virial mass of the Galaxy to be Mvir = 1.2+0.5−0.4 × 1012M⊙ (Kafle et al. 2012). We also show that the rotation curve of the Galaxy has undulations similar to what have also been seen in the studies of the HI gas (Sofue et al. 2009).

scholarly journals The AMBRE Project: Origin and evolution of sulfur in the Milky Way

2021 ◽  
Vol 647 ◽  
pp. A162 ◽  
Author(s):  
J. Perdigon ◽  
P. de Laverny ◽  
A. Recio-Blanco ◽  
E. Fernandez-Alvar ◽  
P. Santos-Peral ◽  
...  
Keyword(s):  
Chemical Evolution ◽  
Chemical Element ◽  
Milky Way ◽  
Point Of View ◽  
Dynamical Study ◽  
Stellar Spectra ◽  
Origin And Evolution ◽  
Stellar Ages ◽  
The Galaxy ◽  
External Galaxies

Context. Sulfur is a volatile chemical element that plays an important role in tracing the chemical evolution of the Milky Way and external galaxies. However, its nucleosynthesis origin and abundance variations in the Galaxy are still unclear because the number of available stellar sulfur abundance measurements is currently rather small. Aims. The goal of the present article is to accurately and precisely study the sulfur content of large number of stars located in the solar neighbourhood. Methods. We use the parametrisation of thousands of high-resolution stellar spectra provided by the AMBRE Project, and combine it with the automated abundance determination GAUGUIN to derive local thermodynamic equilibrium sulfur abundances for 1855 slow-rotating FGK-type stars. This is the largest and most precise catalogue of sulfur abundances published to date. It covers a metallicity domain as high as ∼2.5 dex starting at [M/H] ∼ −2.0 dex. Results. We find that the sulfur-to-iron abundances ratio is compatible with a plateau-like distribution in the metal-poor regime, and then starts to decrease continuously at [M/H] ∼ −1.0 dex. This decrease continues towards negative values for supersolar metallicity stars as recently reported for magnesium and as predicted by Galactic chemical evolution models. Moreover, sulfur-rich stars having metallicities in the range [ − 1.0, −0.5] have very different kinematical and orbital properties with respect to more metal-rich and sulfur-poor ones. Two disc components, associated with the thin and thick discs, are thus seen independently in kinematics and sulfur abundances. The sulfur radial gradients in the Galactic discs have also been estimated. Finally, the enrichment in sulfur with respect to iron is nicely correlated with stellar ages: older metal-poor stars have higher [S/M] ratios than younger metal-rich ones. Conclusions. This work has confirmed that sulfur is an α-element that could be considered to explore the Galactic populations properties. For the first time, a chemo-dynamical study from the sulfur abundance point of view, as a stand-alone chemical element, is performed.

scholarly journals The chemodynamics of prograde and retrograde Milky Way stars

2020 ◽  
Vol 643 ◽  
pp. A69
Author(s):  
Georges Kordopatis ◽  
Alejandra Recio-Blanco ◽  
Mathias Schultheis ◽  
Vanessa Hill
Keyword(s):  
Milky Way ◽  
Galactic Center ◽  
Galactic Plane ◽  
Azimuthal Velocity ◽  
Mixed Population ◽  
Photometric Data ◽  
Rotating Stars ◽  
The Galaxy ◽  
History Of ◽  
Different Populations

Context. The accretion history of the Milky Way is still unknown, despite the recent discovery of stellar systems that stand out in terms of their energy-angular momentum space, such as Gaia-Enceladus-Sausage. In particular, it is still unclear how these groups are linked and to what extent they are well-mixed. Aims. We investigate the similarities and differences in the properties between the prograde and retrograde (counter-rotating) stars and set those results in context by using the properties of Gaia-Enceladus-Sausage, Thamnos/Sequoia, and other suggested accreted populations. Methods. We used the stellar metallicities of the major large spectroscopic surveys (APOGEE, Gaia-ESO, GALAH, LAMOST, RAVE, SEGUE) in combination with astrometric and photometric data from Gaia’s second data-release. We investigated the presence of radial and vertical metallicity gradients as well as the possible correlations between the azimuthal velocity, vϕ, and metallicity, [M/H], as qualitative indicators of the presence of mixed populations. Results. We find that a handful of super metal-rich stars exist on retrograde orbits at various distances from the Galactic center and the Galactic plane. We also find that the counter-rotating stars appear to be a well-mixed population, exhibiting radial and vertical metallicity gradients on the order of ∼ − 0.04 dex kpc−1 and −0.06 dex kpc−1, respectively, with little (if any) variation when different regions of the Galaxy are probed. The prograde stars show a vϕ − [M/H] relation that flattens – and, perhaps, even reverses as a function of distance from the plane. Retrograde samples selected to roughly probe Thamnos and Gaia-Enceladus-Sausage appear to be different populations yet they also appear to be quite linked, as they follow the same trend in terms of the eccentricity versus metallicity space.

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