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We use numerical simulations to model Gaia  DR3 data with the aim of constraining the Milky Way (MW) bar and spiral structure parameters. We show that both the morphology and the velocity field in MW-like galactic disc models are strong functions of time, changing dramatically over a few tens of Myr. This suggests that by finding a good match to the observed radial velocity field, vR (x, y), we can constrain the bar-spiral orientation. Incorporating uncertainties into our models is necessary to match the data; most importantly, a heliocentric distance uncertainty above 10–15 per cent distorts the bar's shape and vR quadrupole pattern morphology, and decreases its apparent angle with respect to the Sun-Galactocentric line. An excellent match to the Gaia   DR3 vR (x, y) field is found for a simulation with a bar length Rb ≈ 3.6 kpc. We argue that the data are consistent with an MW bar as short as ∼3 kpc, for moderate strength inner disc spiral structure (A 2/ A 0 ≈ 0.25) or, alternatively, with a bar length up to ∼5.2 kpc, provided that spiral arms are quite weak (A 2/ A 0 ≈ 0.1), and is most likely in the process of disconnecting from a spiral arm. We demonstrate that the bar angle and distance uncertainty can similarly affect the match between our models and the data – a smaller bar angle (20° instead of 30°) requires smaller distance uncertainty (20 per cent instead of 30 per cent) to explain the observations. Fourier components of the face-on density distribution of our models suggest that the MW does not have strong m = 1 and/or m  = 3 spirals near the solar radius. [ABSTRACT FROM AUTHOR]

We investigate the inner regions of the Milky Way using data from APOGEE and Gaia EDR3. Our inner Galactic sample has more than 26 500 stars within |XGal|< 5 kpc, |YGal|< 3.5 kpc, |ZGal|< 1 kpc, and we also carry out the analysis for a foreground-cleaned subsample of 8000 stars that is more representative of the bulge–bar populations. These samples allow us to build chemo-dynamical maps of the stellar populations with vastly improved detail. The inner Galaxy shows an apparent chemical bimodality in key abundance ratios [α/Fe], [C/N], and [Mn/O], which probe different enrichment timescales, suggesting a star formation gap (quenching) between the high- and low-α populations. Using a joint analysis of the distributions of kinematics, metallicities, mean orbital radius, and chemical abundances, we can characterize the different populations coexisting in the innermost regions of the Galaxy for the first time. The chemo-kinematic data dissected on an eccentricity–|Z|max plane reveal the chemical and kinematic signatures of the bar, the thin inner disc, and an inner thick disc, and a broad metallicity population with large velocity dispersion indicative of a pressure-supported component. The interplay between these different populations is mapped onto the different metallicity distributions seen in the eccentricity–|Z|max diagram consistently with the mean orbital radius and Vϕ distributions. A clear metallicity gradient as a function of |Z|max is also found, which is consistent with the spatial overlapping of different populations. Additionally, we find and chemically and kinematically characterize a group of counter-rotating stars that could be the result of a gas-rich merger event or just the result of clumpy star formation during the earliest phases of the early disc that migrated into the bulge. Finally, based on 6D information, we assign stars a probability value of being on a bar orbit and find that most of the stars with large bar orbit probabilities come from the innermost 3 kpc, with a broad dispersion of metallicity. Even stars with a high probability of belonging to the bar show chemical bimodality in the [α/Fe] versus [Fe/H] diagram. This suggests bar trapping to be an efficient mechanism, explaining why stars on bar orbits do not show a significant, distinct chemical abundance ratio signature. [ABSTRACT FROM AUTHOR]

We use Gaia DR2 astrometric and line-of-sight velocity information combined with two sets of distances obtained with a Bayesian inference method to study the 3D velocity distribution in the Milky Way disc. We search for variations in all Galactocentric cylindrical velocity components (V ϕ, VR , and Vz) with Galactic radius, azimuth, and distance from the disc mid-plane. We confirm recent work showing that bulk vertical motions in the R – z plane are consistent with a combination of breathing and bending modes. In the x – y plane, we show that, although the amplitudes change, the structure produced by these modes is mostly invariant as a function of distance from the plane. Comparing to two different Galactic disc models, we demonstrate that the observed patterns can drastically change in short time intervals, showing the complexity of understanding the origin of vertical perturbations. A strong radial VR gradient was identified in the inner disc, transitioning smoothly from 16 km s−1 kpc−1 at an azimuth of 30° < ϕ < 45° ahead of the Sun-Galactic centre line to −16 km s−1 kpc−1 at an azimuth of −45° < ϕ < −30° lagging the solar azimuth. We use a simulation with no significant recent mergers to show that exactly the opposite trend is expected from a barred potential, but overestimated distances can flip this trend to match the data. Alternatively, using an N -body simulation of the Sagittarius dwarf–Milky Way interaction, we demonstrate that a major recent perturbation is necessary to reproduce the observations. Such an impact may have strongly perturbed the existing bar or even triggered its formation in the last 1–2 Gyr. [ABSTRACT FROM AUTHOR]

We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia astrometric solution (TGAS) catalogue to compute the velocity fields yielded by the radial (VR), azimuthal (Vϕ),and vertical (Vz) components of associated Galactocentric velocity.We search in particular for variation in all three velocity components with distance above and below the disc midplane, as well as how each component of Vz (line-of-sight and tangential velocity projections) modifies the obtained vertical structure. To study the dependence of velocity on proper motion and distance, we use two main samples: a RAVE sample including proper motions from the Tycho-2, PPMXL, and UCAC4 catalogues, and a RAVE-TGAS sample with inferred distances and proper motions from the TGAS and UCAC5 catalogues. In both samples, we identify asymmetries in VR and Vz. Below the plane, we find the largest radial gradient to be ∂VR/∂R = -7.01 ± 0.61 km s-1 kpc-1, in agreement with recent studies. Above the plane, we find a similar gradient with ∂VR/∂R = -9.42 ± 1.77 km s-1 kpc-1. By comparing our results with previous studies, we find that the structure in Vz is strongly dependent on the adopted proper motions. Using the Galaxia Milky Way model, we demonstrate that distance uncertainties can create artificial wave-like patterns. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms. [ABSTRACT FROM AUTHOR]

4MOST is a new wide-field, high-multiplex spectroscopic survey facility for the VISTA telescope of ESO. Starting in 2022, 4MOST will deploy 2400 fibres in a 4.1 square degree field-of-view using a positioner based on the tilting spine principle. In this contribution we give an outline of the major science goals we wish to achieve with 4MOST in the area of Galactic Archeology. The 4MOST Galactic Archeology surveys have been designed to address long-standing and far-reaching problems in Galactic science. They are focused on four major themes: 1) Near-field cosmology tests, 2) Chemo-dynamical characterisation of the major Milky Way stellar components, 3) The Galactic Halo and beyond, and 4) Discovery and characterisation of extremely metal-poor stars. In addition to a top-level description of the Galactic surveys we provide information about how the community will be able to join 4MOST via a call for Public Spectroscopic Surveys that ESO will launch. [ABSTRACT FROM AUTHOR]

In the last few decades, many efforts have been made to understand the effect of spiral arms on the gas and stellar dynamics in the Milky Way disc. One of the fundamental parameters of the spiral structure is its angular velocity, or pattern speed Ωp, which determines the location of resonances in the disc and the spirals' radial extent. The most direct method for estimating the pattern speed relies on backward integration techniques, trying to locate the stellar birthplace of open clusters. Here, we propose a new method based on the interaction between the spiral arms and the stars in the disc. Using a sample of around 500 open clusters from the New Catalogue of Optically Visible Open Clusters and Candidates, and a sample of 500 giant stars observed by Apache Point Observatory Galactic Evolution Experiment, we find Ωp = 23.0 ± 0.5 km s-1 kpc-1, for a local standard of rest rotation V0 = 220-km s-1 and solar radius R0 = 8.0 kpc. Exploring a range in V0 and R0 within the acceptable values, 200-240 km s-1 and 7.5-8.5 kpc, respectively, results only in a small change in our estimate of Ωp, that is within the error. Our result is in close agreement with a number of studies which suggest values in the range 20-25 km s-1 kpc-1. An advantage of our method is that we do not need knowledge of the stellar age, unlike in the case of the birthplace method, which allows us to use data from large Galactic surveys. The precision of our method will be improved once larger samples of disc stars with spectroscopic information will become available thanks to future surveys such as 4MOST. [ABSTRACT FROM AUTHOR]

Non-axisymmetries in the Galactic potential (spiral arms and bar) induce kinematic groups such as the Hercules stream. Assuming that Hercules is caused by the effects of the outer Lindblad resonance of the Galactic bar, we model analytically its properties as a function of position in the Galaxy and its dependence on the bar's pattern speed and orientation. Using data from the RAVE survey we find that the azimuthal velocity of the Hercules structure decreases as a function of Galactocentric radius, in a manner consistent with our analytical model. This allows us to obtain new estimates of the parameters of the Milky Way's bar. The combined likelihood function of the bar's pattern speed and angle has its maximum for a pattern speed of Ωb = (1.89 ± 0.08) × Ω0, where Ω0 is the local circular frequency. Assuming a solar radius of 8.05 kpc and a local circular velocity of 238 km s-1, this corresponds to Ωb = 56 ± 2kms-1 kpc-1. On the other hand, the bar's orientation ϕb cannot be constrained with the available data. In fact, the likelihood function shows that a tight correlation exists between the pattern speed and the orientation, implying that a better description of our best fit results is given by the linear relation Ωb/Ω0 = 1.91+0.0044 (ϕb(deg) - 48), with standard deviation of 0.02. For example, for an angle of ϕb = 30 deg the pattern speed is 54.0 ± 0.5kms-1 kpc-1. These results are not very sensitive to the other Galactic parameters such as the circular velocity curve or the peculiar motion of the Sun, and are robust to biases in distance. [ABSTRACT FROM AUTHOR]

Non-axisymmetric components, such as spirals and central bars, play a major role in shaping galactic discs. An important aspect of the disc secular evolution driven by these perturbers is the radial migration of stars. It has been suggested recently that migration can populate a thick-disc component from inner-disc stars with high vertical energies. Since this has never been demonstrated in simulations, we study in detail the effect of radial migration on the disc velocity dispersion and disc thickness, by separating simulated stars into migrators and non-migrators. We apply this method to three isolated barred Tree-SPH N-body galaxies with strong radial migration. Contrary to expectations, we find that as stellar samples migrate, on the average, their velocity dispersion change (by as much as 50%) in such a way as to approximately match the non-migrating population at the radius at which they arrive. We show that, in fact, migrators suppress heating in parts of the disc. To confirm the validity of our findings, we also apply our technique to three cosmological re-simulations, which use a completely different simulation scheme and, remarkably, find very similar results. We believe the inability of migration to thicken discs is a fundamental property of internal disc evolution, irrespective of the migration mechanism at work. We explain this with the approximate conservation of the (average) vertical and radial actions rather than the energy. This "action mixing" can be used to constrain the migration rate in the Milky Way: estimates of the average vertical action in observations for different populations of stars should reveal flattening with radius for older groups of stars. [ABSTRACT FROM AUTHOR]

Context. NGC6791 is a unique stellar system among Galactic open clusters, which is at the same time one of the oldest open clusters and the most metal rich. It is located inside the solar circle, harbors a large population of binary stars, and possibly experienced prolonged star formation. The combination of all these properties is puzzling and poses the intriguing question of its origin. Aims. One possible scenario is that the cluster formed close to the Galactic Center and later migrated outward to its current location. In this work we study the cluster's orbit and investigate the possible migration processes that may have displaced NGC 6791 to its present-day position, under the assumption that it actually formed in the inner disk. Methods. To this aim we performed integrations of NGC6791's orbit in a potential consistent with the main Milky Way parameters. In addition to analytical expressions for halo, bulge and disk, we also consider the effect of bar and spiral arm perturbations, which are expected to be very important for the disk dynamical evolution, especially inside the solar circle. Starting from state-of-the art initial conditions for NGC6791, we calculated 1000 orbits back in time for about 1Gyr turning different non-axisymmetric components of the global potential on and off. We then compared statistical estimates of the cluster's recent orbital parameters with the orbital parameters of 104 test-particles originating close to the Galactic Center (with initial galocentric radii in the range of 3-5 kpc) and undergoing radial migration during 8Gyr of forward integration. Results. We find that a model that incorporates a strong bar and spiral arm perturbations can indeed be responsible for the migration of NGC6791 from the inner disk (galocentric radii of 3-5 kpc) to its present-day location. Such a model can provide orbital parameters that are close enough to the observed ones. However, the probability of this scenario as it results from our investigations is very low. [ABSTRACT FROM AUTHOR]

We find that a steady-state bar induces transient features at low velocities in the solar neighbourhood velocity distribution due to the initial response of the disc, following the formation of the bar. We associate these velocity streams with two quasi-periodic orbital families, librating around the stable and orbits near the bar's outer Lindblad resonance. In a reference frame moving with the bar, these otherwise stationary orbits precess on a time-scale dependent on the strength of the bar, consistent with predictions from a simple Hamiltonian model for the resonance. This behaviour allows the two orbital families to reach the solar neighbourhood and manifest themselves as clumps in the u– v plane moving away from , and towards the Galactic centre. Depending on the bar parameters and time since its formation, this model is consistent with the Pleiades and Coma Berenices, or Pleiades and Sirius moving groups seen in the Hipparcos stellar velocity distribution, if the Milky Way bar angle is and its pattern speed is , where is the angular velocity of the local standard of rest. Since the process is recurrent, we can achieve a good match about every six local standard of rest rotations. However, to be consistent with the fraction of stars in the Pleiades, we estimate that the Milky Way bar formed ∼2 Gyr ago. This model argues against a common dynamical origin for the Hyades and Pleiades moving groups. [ABSTRACT FROM AUTHOR]

We investigate the effect of spiral structure on the Galactic disc as viewed by pencil beams centred on the Sun, relevant to upcoming surveys such as ARGOS, SEGUE and GAIA. Using a steady-state spiral structure model, we create synthetic Galactic maps which we call pencil-beam maps (PBMs) of the following observables: line-of-sight velocities, the corresponding velocity dispersion and the stellar number density that are functions of distance from the observer. We show that PBMs are a powerful tool for analysing spiral patterns but the uncertainty of the Milky Way structure prevents their simple interpretation. For the case of steady-state spiral structure, we show that such maps can be used to infer spiral structure parameters, such as pattern speed, solar phase angle and number of arms. The mean line-of-sight velocity and velocity dispersion are affected by up to , which is well within the detectable limit for forthcoming radial velocity surveys. One can measure the pattern speed by searching for imprints of resonances. In the case of a two-armed spiral structure it can be inferred from the radius of a high velocity dispersion ring situated at the 2:1 inner Lindblad resonance (ILR). This information, however, must be combined with information related to the velocities and stellar number density in order to distinguish from a four-armed structure. If the pattern speed is such that the 2:1 ILR is hidden inside the Galactic bulge, the 2:1 outer Lindblad resonance will be present in the outer Galaxy and thus can equivalently be used to estimate the pattern speed. Once the pattern speed is known, the solar angle can be estimated from the line-of-sight velocities and the number density PBMs. The models presented here are preliminary as transient spiral structure models should also be considered. Forthcoming radial velocity surveys are likely to provide powerful constraints of the structure of the Milky Way disc. [ABSTRACT FROM AUTHOR]

Using data from the Radial Velocity Experiment (RAVE) and the Tycho- Gaia astrometric solution catalogue (TGAS), we study the vertical velocity (Vz) patterns in the Milky Way disc. We search in particular for variation in velocity with distance above and below the disc midplane. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms. [ABSTRACT FROM AUTHOR]