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A recently discovered young, high-velocity giant star J01020100-7122208 is a good candidate of hypervelocity star ejected from the Galactic center, although it has a bound orbit. If we assume that this star was ejected from the Galactic center, it can be used to constrain the Galactic potential, because the deviation of its orbit from a purely radial orbit informs us of the torque that this star has received. Based on this assumption, we estimate the flattening of the Galactic dark matter halo by using the Gaia DR2 data and the circular velocity data. Our Bayesian analysis shows that the orbit of J01020100-7122208 favors a prolate halo within ~ 10 kpc from the Galactic center. The posterior distribution of the density flattening q shows a broad distribution at q ≳ 1 and peaks at q ≃ 1.5. Also, 98.5% of the posterior distribution is located at q > 1, highly disfavoring an oblate halo. [ABSTRACT FROM AUTHOR]

In the efforts to map the Milky Way structure, the central regions have remained very difficult to probe. The VISTA Variables in the Vía Láctea Survey (VVV) is a near-IR variability Survey that scans 560 sq.deg. across the Milky Way bulge and an adjacent section of the southern mid-plane. The main goal of the VVV Survey is to build a 3D map of the structure of the inner Galaxy and characterize its stellar populations. This survey has discovered different kinds of objects, such as globular clusters, Microlensing events, RR Lyrae stars, Cepheids, WITs, among others. The extension of the Survey (VVVX) is observing until 2020, tripling the areal coverage, and complementing the variability studies done by the VVV Survey. [ABSTRACT FROM AUTHOR]

We have collected 2330 Cepheids to establish an intuitive 3D map of the Milky Way's disk. As regards the warp amplitude, the Cepheid disk agrees well with the gas disk for radii up to 15 kpc. However, the mean line of nodes (LON) of the Cepheid disk deviates from the Galactic Center–Sun direction by 17.5±1.0°. This is a new and different result compared with previous results. The LON is not stable at any given radius, but it twists. The twisted pattern suggests that the formation of the Milky Way's warp is dominated by the massive inner disk. The kinematic warp defined by the Cepheids is also in concordance with the spatial warp. In the 2020 era, the anticipated increasing number of new Cepheids will provide a key opportunity to view our Milky Way's disk as a whole, and we expect that our knowledge of the disk's main structural features will be much improved. [ABSTRACT FROM AUTHOR]

We present new results on the Galactic bar/bulge transverse velocity structure using Gaia and the VISTA Variables in Via Lactea (VVV) survey. Gaia is complemented in high extinction regions by the multi-epoch infrared VVV observations for which derived relative proper motions can be tied to Gaia's absolute frame. We extract kinematic maps (both 2D and 3D) of the Galactic bar/bulge, from which we measure the pattern speed of the bar using a novel technique. We focus on the evidence of an X-shaped bulge from the kinematic maps. [ABSTRACT FROM AUTHOR]

The unprecedented amount and accuracy of kinematic data from the second release of the Gaia mission have started revolutionizing our understanding of the dynamics of the Milky Way disk. The detailed stellar velocity field in the Galactic disk should allow us to constrain with unprecedented precision the parameters of the non-axisymmetric modes of the disk. We present here the status of our current modelling efforts in this area, and their implication on the dynamics of the Galactic bar in particular. [ABSTRACT FROM AUTHOR]

Classical Cepheids and RR Lyrae-type variable stars are widely-used tracers of young (< 300 Myr) and old (> 10 Gyr) stellar populations, respectively. These stars also serve as distance indicators allowing for Galactic structure studies. Robust detection of pulsating variables requires precise and relatively frequent observations over several years. Recently, the OGLE survey has discovered nearly 1,300 new genuine classical Cepheids and 15,000 RR Lyrae stars along the southern Galactic plane. Here, we present the picture of the Milky Way's thin disk drawn with the Cepheids and the view of the Galactic old population that emerges from the set of known RR Lyrae stars. [ABSTRACT FROM AUTHOR]

Stellar surveys and dynamical models have recently led to important progress on understanding the dynamical structure of the Milky Way's bar and central box/peanut bulge. This talk briefly reviews the density structure of the bulge and bar from star count tomography, the cylindrical rotation of bulge stars, and the measurements of their stellar masses and pattern speed that have been obtained by fitting dynamical models to the combined star count and line-of-sight velocity data. Recent work deriving absolute proper motions throughout the bulge from the VIRAC survey and Gaia has led to a new 3D measurement of the barred bulge kinematics which is expected to greatly improve the dynamical models, and has already confirmed the relatively slow pattern speed (∼40 kms−1 kpc−1) obtained from the previous dynamical and gas-dynamical modelling. [ABSTRACT FROM AUTHOR]

In order to study the most reddened areas of the Milky Way we used near-IR data from the VVV Survey. For the first time, the VISTA telescope allows us to observe the mid-plane through the Galactic bulge and study the disk in the other side of the Milky Way. Motivated by the detection of hundreds of microlensing events in the inner regions of the Galaxy, we propose three new configurations of microlensing events, placing the sources in the far-disk and the lenses in the far-disk/bulge/near-disk. These new configurations will change the usual way to interpret the timescale distributions due to the different populations along the line of sight, that exhibit varied transverse velocities and relative distances. [ABSTRACT FROM AUTHOR]

The on-going phase mixing in the vertical direction of the Galactic disk has been discovered with the revolutionary Gaia DR2 data. It manifests itself as the snail shell in the Z–Vz phase space. To better understand the origin and properties of the phase mixing process, we study the phase-mixing signatures in moving groups (also known as the kinematic streams) with the Gaia DR2 data in the Galactic disk near the Solar circle. Interestingly, the phase space snail shell exists only in the main kinematic streams with |VR|≲ 50 km/s and |Vφ –VLSR|≲30 km/s, i.e., stars on dynamically "colder" orbits. Compared to the colder orbits, the hotter orbits may have phase-wrapped away already due to the much larger dynamical range in radial variation to facilitate faster phase mixing. These results help put tighter constraints on the vertical perturbation history of the Milky Way disk. To explain the lack of a well-defined snail shell in the hotter orbits, the disk should have been perturbed at least ∼400–500 Myr ago. Our results offer more support to the recent satellite-disk encounter scenario than the internal bar buckling perturbation scenario as the origin of the phase space mixing. [ABSTRACT FROM AUTHOR]

We have derived absolute proper motions of stars in the Galactic bulge region combining the VVV InfraRed Astrometric Catalogue (VIRAC) and Gaia. We use the proper motions to study the kinematic structure of the bulge both integrated along the line-of-sight and in magnitude intervals using red clump stars as standard candles. In parallel we compare to a made-to-measure barred dynamical model, folding in the VIRAC selection function, to understand and interpret the structures that we observe. The barred dynamical model, which contains a boxy/peanut bulge, and has a pattern speed of 37.5 kms−1 kpc−1, is able to reproduce all structures impressively well. [ABSTRACT FROM AUTHOR]

Whiting 1 is a faint and young globular cluster embedded in the Sag dSph. It has similar distance, metalicity and radial velocity with the trailing stream of the Sag. Due to these special properties, Whiting 1 was suggested to be associated with the trailing stream of Sag. However, its origin is still unclear and further investigation of its relation with Sgr dSph is needed. In this work, we use DECaLS data to search the tidal tail of Whiting 1, with the aim of looking for spatial connection between Whiting 1 and Sgr dSph. With Matched Filter method, we detected a tidal tail around the main body of Whiting 1. This tail extends to at least 0.5 degree and aligns with the mean orbit direction of Sgr dSph. This tail is newly detected and it provides additional evidence of the association between Whiting 1 and Sgr trailing stream. [ABSTRACT FROM AUTHOR]