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Disc stars from the Gaia DR3 RVS catalogue are selected to explore the phase spiral in the Galaxy. The data reveal a two-armed phase spiral pattern in the local z – vz plane inside the solar radius, which appears clearly when colour-coded by 〈 v R〉(z, vz): this is characteristic of a breathing mode that can in principle be produced by in-plane non-axisymmetric perturbations. The phase spiral pattern becomes single armed outside the solar radius. When a realistic analytic model with a steadily rotating bar and 2-armed spiral arms as perturbation is used to perform particle test integrations, the pseudo-stars get a prominent spiral pattern in the 〈 v R〉 map in the x–y plane. Additionally, clear breathing mode evidence at a few |$\, \mathrm{km\, s}^{-1}$| level can be seen in the 〈 vz 〉 map on the x–z plane, confirming that such breathing modes are non-negligible in the joint presence of a bar and spiral arms. However, no phase spiral is perceptible in the (z, vz) plane. When an initial vertical perturbation is added to carry out the simulation, the one-armed phase spirals can clearly be seen 500 Myr after the perturbation and gradually disappear inside out. Finally, we show as a proof of concept how a toy model of a time-varying non-axisymmetric in-plane perturbation with varying pattern speed can produce a strong two-armed phase spiral. We conclude a time-varying strong internal perturbation together with an external vertical perturbation could perhaps explain the transition between the two-armed and one-armed phase spirals around the solar radius. [ABSTRACT FROM AUTHOR]

Aims. The most metal-poor tail of the Milky Way ([Fe/H] ≤ −2.5) contains a population of stars on very prograde planar orbits, whose origins and evolution remain puzzling. One possible scenario is that they are shepherded by the bar from the inner Galaxy, where many of the old and low-metallicity stars in the Galaxy are located. Methods. To investigate this scenario, we used test-particle simulations with an axisymmetric background potential plus a central bar model. The test particles were generated by an extended distribution function (EDF) model based on the observational constraints of bulge stars. Results. According to the simulation results, a bar with a constant pattern speed is not efficient in terms of helping bring stars from the bulge to the solar vicinity. In contrast, when the model includes a decelerating bar, some bulge stars can gain rotation and move outwards as they are trapped in the bar's resonance regions. The resulting distribution of shepherded stars heavily depends on the present-day azimuthal angle between the bar and the Sun. The majority of the low-metallicity bulge stars driven outwards are distributed in the first and fourth quadrants of the Galaxy with respect to the Sun and about 10% of them are within 6 kpc from us. Conclusions. Our experiments indicate that the decelerating bar perturbation can be a contributing mechanism that may partially explain the presence of the most metal-poor stars with prograde planar orbits in the Solar neighbourhood, but it is unlikely to be the only one. [ABSTRACT FROM AUTHOR]

Both the three-dimensional density of red clump giants and the gas kinematics in the inner Galaxy indicate that the pattern speed of the Galactic bar could be much lower than previously estimated. Here, we show that such slow bar models are unable to reproduce the bimodality observed in local stellar velocity space. We do so by computing the response of stars in the solar neighbourhood to the gravitational potential of slow and fast bars, in terms of their perturbed distribution function in action-angle space up to second order, as well as by identifying resonantly trapped orbits. We also check that the bimodality is unlikely to be produced through perturbations from spiral arms, and conclude that, contrary to gas kinematics, local stellar kinematics still favour a fast bar in the Milky Way, with a pattern speed of the order of almost twice (and no less than 1.8 times) the circular frequency at the Sun's position. This leaves open the question of the nature of the long flat extension of the bar in the Milky Way. [ABSTRACT FROM AUTHOR]

Starting from an axisymmetric equilibrium distribution function (DF) in action space, representing a Milky Way thin disc stellar population, we use the linearized Boltzmann equation to explicitly compute the response to a three-dimensional spiral potential in terms of the perturbed DF. This DF, valid away from the main resonances, allows us to investigate a snapshot of the velocity distribution at any given point in three-dimensional configuration space. Moreover, the first-order moments of the DF give rise to non-zero radial and vertical bulk flows - namely breathing modes - qualitatively similar to those recently observed in the extended solar neighbourhood. We show that these analytically predicted mean stellar motions are in agreement with the outcome of test-particle simulations. Moreover, we estimate for the first time the reduction factor for the vertical bulk motions of a stellar population compared to the case of a cold fluid. Such an explicit expression for the full perturbed DF of a thin disc stellar population in the presence of spiral arms will be helpful in order to dynamically interpret the detailed information on the Milky Way disc stellar kinematics that will be provided by upcoming large astrometric and spectroscopic surveys of the Galaxy. [ABSTRACT FROM AUTHOR]

Evidence for non-zero mean stellar velocities in the direction perpendicular to the Galactic plane has been accumulating from various recent large spectroscopic surveys. Previous analytical and numerical work has shown that a 'breathing mode' of the Galactic disc, similar to what is observed in the solar vicinity, can be the natural consequence of a non-axisymmetric internal perturbation of the disc. Here we provide a general analytical framework, in the context of perturbation theory, allowing us to compute the vertical bulk motions generated by a single internal perturber (bar or spiral pattern). In the case of the Galactic bar, we show that these analytically predicted bulk motions are well in line with the outcome of a numerical simulation. The mean vertical motions induced by the Milky Way bar are small (mean velocity of less than 1 km s-1) and cannot be responsible alone for the observed breathing mode, but they are existing. Our analytical treatment is valid close to the plane for all the non-axisymmetric perturbations of the disc that can be described by small-amplitude Fourier modes. Further work should study how the coupling of multiple internal perturbers and external perturbers is affecting the present analytical results. [ABSTRACT FROM AUTHOR]

Evidence for non-zero mean stellar velocities in the direction perpendicular to the Galactic plane has been accumulating from various recent large spectroscopic surveys. Previous analytical and numerical work has shown that a 'breathing mode' of the Galactic disc, similar to what is observed in the solar vicinity, can be the natural consequence of a non-axisymmetric internal perturbation of the disc. Here we provide a general analytical framework, in the context of perturbation theory, allowing us to compute the vertical bulk motions generated by a single internal perturber (bar or spiral pattern). In the case of the Galactic bar, we show that these analytically predicted bulk motions are well in line with the outcome of a numerical simulation. The mean vertical motions induced by the Milky Way bar are small (mean velocity of less than 1 kms-1) and cannot be responsible alone for the observed breathing mode, but they are existing. Our analytical treatment is valid close to the plane for all the non-axisymmetric perturbations of the disc that can be described by small-amplitude Fourier modes. Further work should study how the coupling of multiple internal perturbers and external perturbers is affecting the present analytical results. [ABSTRACT FROM AUTHOR]

We present maps of the stellar streams detected in the Gaia Data Release 2 (DR2) and Early Data Release 3 (EDR3) catalogs using the STREAMFINDER algorithm. We also report the spectroscopic follow-up of the brighter DR2 stream members obtained with the high-resolution CFHT/ESPaDOnS and VLT/UVES spectrographs as well as with the medium-resolution NTT/EFOSC2 spectrograph. Two new stellar streams that do not have a clear progenitor are detected in DR2 (named Hríd and Gunnthrá), and seven are detected in EDR3 (named Gaia-6 to Gaia-12). Several candidate streams are also identified. The software also finds very long tidal tails associated with the 15 globular clusters: NGC 288, NGC 1261, NGC 1851, NGC 2298, NGC 2808, NGC 3201, M68, ωCen, NGC 5466, Palomar 5, M5, NGC 6101, M92, NGC 6397, and NGC 7089. These stellar streams will be used in subsequent contributions in this series to chart the properties of the Galactic acceleration field on ∼100 pc to ∼100 kpc scales. [ABSTRACT FROM AUTHOR]

We explore the connections between stellar age, chemistry, and kinematics across a Galacto- centric distance of 7.5 < R(kpc) < 9.0, using a sample of ~12 000 intermediate-mass (FGK) turn-off stars observed with the RAdial Velocity Experiment (RAVE) survey. The kinematics of this sample are determined using radial velocity measurements from RAVE, and parallax and proper motion measurements from the Tycho--Gaia Astrometric Solution (TGAS). In addition, ages for RAVE stars are determined using a Bayesian method, taking TGAS parallaxes as a prior. We divide our sample into young (0 < τ < 3 Gyr) and old (8 < τ < 13 Gyr) populations, and then consider different metallicity bins for each of these age groups.We ?nd signi?cant differences in kinematic trends of young and old, metal-poor and metal-rich, stellar populations. In particular, we ?nd a strong metallicity dependence in the mean Galactocentric radial velocity as a function of radius (∂VR/∂R) for young stars, with metal-rich stars having a much steeper gradient thanmetal-poor stars. For ∂Vφ/∂R, young, metal-rich stars signi?cantly lag the LSR with a slightly positive gradient, while metal-poor stars show a negative gradient above the LSR.We interpret these ?ndings as correlations between metallicity and the relative contributions of the non-axisymmetries in the Galactic gravitational potential (the spiral arms and the bar) to perturb stellar orbits. [ABSTRACT FROM AUTHOR]

We report on the detection of a small overdensity of stars in velocity space with systematically higher Galactocentric rotation velocity than the Sun by about 20 km s-1 in the Gaia Data Release 1 Tycho-Gaia astrometric solution data. We find these fast Galactic rotators more clearly outside of the Solar radius, compared to inside of the Solar radius. In addition, the velocity of the fast Galactic rotators is independent of the Galactocentric distance up to R - R0 ~ 0.6 kpc. Comparing with numerical models, we qualitatively discuss that a possible cause of this feature is the co-rotation resonance of the Perseus spiral arm, where the stars in the peri-centre phase in the trailing side of the Perseus spiral arm experience an extended period of acceleration owing to the torque from the Perseus arm. [ABSTRACT FROM AUTHOR]

We investigate the kinematic signatures induced by spiral and bar structure in a set of simulations of Milky Way-sized spiral disc galaxies. The set includes test particle simulations that follow a quasi-stationary density wave-like scenario with rigidly rotating spiral arms, and Nbody simulations that host a bar and transient, corotating spiral arms. From a location similar to that of the Sun, we calculate the radial, tangential and line-of-sight peculiar velocity fields of a patch of the disc and quantify the fluctuations by computing the power spectrum from a two-dimensional Fourier transform. We find that the peculiar velocity power spectrum of the simulation with a bar and transient, corotating spiral arms fits very well to that of APOGEE red clump star data, while the quasi-stationary density wave spiral model without a bar does not. We determine that the power spectrum is sensitive to the number of spiral arms, spiral arm pitch angle and position with respect to the spiral arm. However, it is necessary to go beyond the line-of-sight velocity field in order to distinguish fully between the various spiral models with this method. We compute the power spectrum for different regions of the spiral discs, and discuss the application of this analysis technique to external galaxies. [ABSTRACT FROM AUTHOR]

The article focuses on the creation of three dimensional (3D) maps of diffuse interstellar bands (DIBs). Topics include the spatial distribution of interstellar dust, the study of optical and near-infrared (NIR) wavelengths, and the interstellar medium (ISM). Information is provided on modeling of the results of the Radial Velocity Experiment (RAVE) spectroscopic survey.

We report the discovery of eight lithium-rich field giants found in a high-resolution spectroscopic sample of over 700 metal-poor stars ([Fe/H] < -0.5) selected from the Radial Velocity Experiment survey. The majority of the Li-rich giants in our sample are very metal-poor ([Fe/H] ≲ -1.9), and have a Li abundance (in the form of 7Li), A(Li) = log(n(Li)/n(H)) + 12, between 2.30 and 3.63, well above the typical upper red giant branch (RGB) limit, A(Li) < 0.5, while two stars, with A(Li) ~ 1.7-1.8, show similar lithium abundances to normal giants at the same gravity. We further included two metal-poor, Li-rich globular cluster giants in our sample, namely the previously discovered M3-IV101 and newly discovered (in this work) M68-A96. This comprises the largest sample of metal-poor Li-rich giants to date. We performed a detailed abundance analysis of all stars, finding that the majority of our sample stars have elemental abundances similar to that of Li-normal halo giants. Although the evolutionary phase of each Li-rich giant cannot be definitively determined, the Li-rich phase is likely connected to extra mixing at the RGB bump or early asymptotic giant branch that triggers cool bottom processing in which the bottom of the outer convective envelope is connected to the H-burning shell in the star. The surface of a star becomes Li-enhanced as 7Be (which burns to 7Li) is transported to the stellar surface via the Cameron-Fowler mechanism. We discuss and discriminate among several models for the extra mixing that can cause Li production, given the detailed abundances of the Li-rich giants in our sample. [ABSTRACT FROM AUTHOR]

We have undertaken the study of the elemental abundances and kinematic properties of a metal-poor sample of candidate thick-disk stars selected from the Radial Velocity Experiment spectroscopic survey of bright stars to differentiate among the present scenarios of the formation of the thick disk. In this paper, we report on a sample of 214 red giant branch, 31 red clump/horizontal branch, and 74 main-sequence/sub-giant branch metal-poor stars, which serves to augment our previous sample of only giant stars. We find that the thick disk [α/Fe] ratios are enhanced and have little variation (<0.1 dex), in agreement with our previous study. The augmented sample further allows, for the first time, investigation of the gradients in the metal-poor thick disk. For stars with [Fe/H] < - 1.2, the thick disk shows very small gradients, <0.03 ± 0.02 dex kpc-1, in α-enhancement, while we find a +0.01 ± 0.04 dex kpc-1 radial gradient and a -0.09 ± 0.05 dex kpc-1 vertical gradient in iron abundance. In addition, we show that the peak of the distribution of orbital eccentricities for our sample agrees better with models in which the stars that comprise the thick disk were formed primarily in the Galaxy, with direct accretion of stars contributing little. Our results thus disfavor direct accretion of stars from dwarf galaxies into the thick disk as a major contributor to the thick-disk population, but cannot discriminate between alternative models for the thick disk, such as those that invoke high-redshift (gas-rich) mergers, heating of a pre-existing thin stellar disk by a minor merger, or efficient radial migration of stars. [ABSTRACT FROM AUTHOR]

Globular clusters (GCs) have proven to be essential to our understanding of many important astrophysical phenomena. Here, we analyse spectroscopic observations of 10 halo GCs to determine their dark matter (DM) content, their tidal heating by the Galactic disc and halo, describe their metallicities and the likelihood that Newtonian dynamics explains their kinematics. We analyse a large number of members in all clusters, allowing us to address all these issues together, and we have included NGC 288 and M30 to overlap with previous studies. We find that any flattening of the velocity dispersion profiles in the outer regions of our clusters can be explained by tidal heating. We also find that all our GCs have M/L , therefore, we infer the observed dynamics do not require DM, or a modification of gravity. We suggest that the lack of tidal heating signatures in distant clusters indicates the halo is not triaxial. The isothermal rotations of each cluster are measured, with M4 and NGC 288 exhibiting rotation at a level of and , respectively. We also indirectly measure the tidal radius of NGC 6752, determining a more realistic figure for this cluster than current literature values. Lastly, an unresolved and intriguing puzzle is uncovered with regard to the cooling of the outer regions of all ten clusters. [ABSTRACT FROM AUTHOR]

Globular clusters (GCs) are an important test bed for Newtonian gravity in the weak-acceleration regime, which is vital to our understanding of the nature of the gravitational interaction. Recent claims have been made that the velocity dispersion profiles of GCs flatten out at large radii, despite an apparent paucity of dark matter (DM) in such objects, indicating the need for a modification of gravitational theories. We continue our investigation of this claim, with the largest spectral samples ever obtained of 47 Tucanae (47 Tuc) and M55. Furthermore, this large sample allows for an accurate metallicity calibration based on the equivalent widths of the calcium triplet lines and K-band magnitude of the Tip of the Red Giant Branch. Assuming an isothermal distribution, the rotations of each cluster are also measured with both clusters exhibiting clear rotation signatures. The global velocity dispersions of NGC 121 and Kron 3, two GCs in the Small Magellanic Cloud, are also calculated. By applying a simple dynamical model to the velocity dispersion profiles of 47 Tuc and M55, we calculate their mass-to-light profiles, total masses and central velocity dispersions. We find no statistically significant flattening of the velocity dispersion at large radii for M55, and a marked increase in the profile of 47 Tuc for radii greater than approximately half the tidal radius. We interpret this increase as an evaporation signature, indicating that 47 Tuc is undergoing, or has undergone, core-collapse, but find no requirement for DM or a modification of gravitational theories in either cluster. [ABSTRACT FROM AUTHOR]

Testing Newtonian gravity in the weak-acceleration regime is vital to our understanding of the nature of the gravitational interaction. It has recently been claimed that the velocity dispersion profiles of several globular clusters flatten out at large radii, reminiscent of galaxy rotation curves, even though globular clusters are thought to contain little or no dark matter. We investigate this claim, using AAOmega observations of four globular clusters, namely M22, M30, M53 and M68. M30, one such cluster that has had this claim made for its velocity dispersion, was included for comparison with previous studies. We find no statistically significant flattening of the velocity dispersion at large radii for any of our target clusters and, therefore, we infer that the observed dynamics do not require that globular clusters are dark matter dominated, or a modification of gravity. Furthermore, by applying a simple dynamical model we determine the radial mass-to-light profiles for each cluster. The isothermal rotations of each cluster are also measured, with M22 exhibiting clear rotation, M68 possible rotation and M30 and M53 lacking any rotation, within the uncertainties. [ABSTRACT FROM AUTHOR]