The recent discovery of a spiral feature in the Z − V Z phase plane in the solar neighborhood implies that the galactic disk has been remarkably affected by a dwarf galaxy passing through it some hundreds of millions of years ago. Using 429,500 Large Sky Area Multi-Object Fibre Spectroscopic Telescope K giants stars, we show that the spiral feature exists not only in the solar vicinity but it also extends to about 15 kpc from the Galactic center and then disappears beyond this radius. Moreover, we find that when the spiral features in a plot of V ϕ as a function of position in the Z − V Z plane at various galactocentric radii are remapped to the R − Z plane, the spiral can explain well the observed asymmetric velocity substructures. This is evidence that the phase spiral features are the same as the bulk motions found in previous work as well as this work. Test particle simulations and N-body simulations show that an encounter with a dwarf galaxy a few hundred million years ago will induce a perturbation in the galactic disk. In addition, we find that the last impact of Sgr dSph can also contribute to the flare. As a consequence of the encounter, the distribution function of disk stars at a large range of radii is imprinted by the gravitational perturbation.

We investigate the morphology and kinematics of the Galactic spiral structure based on a new sample of O-and early B-type stars. We select 6,858 highly confident OB star candidates from the combined data of the VST Photometric Hα Survey Data Release 2 (VPHAS+ DR2) and the Gaia Data Release 2 (Gaia DR2). Together with the O-B2 stars from the literature, we build a sample consisting of 14,880 O-and early B-type stars, all with Gaia parallax uncertainties smaller than 20 per cent. The new sample, hitherto the largest one of O-and early B-type stars with robust distance and proper motion estimates, covers the Galactic plane of distances up to " 6 kpc from the Sun. The sample allows us to examine the morphology of the Scutum, Sagittarius, Local and Perseus Arms in great detail. The spiral structure of the Milky Way as traced by O-and early B-type stars shows flocculent patterns. Accurate structure parameters, as well as the means and dispersions of the vertical velocity distributions of the individual spiral arms are presented.

We perform analysis of the three-dimensional kinematics of Milky Way disk stars in mono-age populations. We focus on stars between Galactocentric distances of R = 6 and 14 kpc, selected from the combined LAMOST DR4 red clump giant stars and Gaia DR2 proper motion catalogue. We confirm the 3D asymmetrical motions of recent works, and we provide time tagging of the Galactic outer disk asymmetrical motions near the anticenter direction out to Galactocentric distances of 14 kpc. Radial Galactocentric motions reach values up to 10 km s −1 , depending on the age of the population, and present a north-south asymmetry in the region corresponding to density and velocity substructures that were sensitive to the perturbations in the early 6 Gyr. After that time, the disk stars of this structure are becoming older and kinematically hotter and not sensitive to the possible perturbations, and we find it is a low α, metal rich, relatively younger population. With the quantitative analysis, we find stars both above and below the plane at R 9 kpc exhibit bending mode motions of which the sensitive duration is around 8 Gyr. Some possible scenarios for these asymmetries are discussed, including a fast rotating bar, spiral arms, minor mergers, sub-halos, warp dynamics, and streams. Although we cannot rule out other factors, for the current results, we speculate that the in-plane asymmetries might be mainly caused by gravitational attraction of overdensities in a spiral arm or monolithic collapse of isolated self-gravitating overdensities from out-of-equilibrium systems. Vertical motions might be dominated by bending and breathing modes induced by inner or external perturbers.

We present a large, homogeneous catalogue of molecular clouds within 4 kpc from the Sun at low Galactic latitudes (|b| ă 10˝) with unprecedented accurate distance determinations. Based on the three-dimensional dust reddening map and estimates of colour excesses and distances of over 32 million stars presented in Chen et al, we have identified 567 dust/molecular clouds with a hierarchical structure identification method and obtained their distance estimates by a dust model fitting algorithm. The typical distance uncertainty is less than 5 per cent. As far as we know, this is the first large catalogue of molecular clouds in the Galactic plane with distances derived in a direct manner. The clouds are seen to lie along the Sagittarius, Local and Perseus Arms. In addition to the known structures, we propose the existence of a possible spur, with a pitch angle of about 34˝, connecting the Local and the Sagittarius Arms in the fourth quadrant. We have also derived the physical properties of those molecular clouds. The distribution of cloud properties in different parameter spaces agrees grossly with the previous results. Our cloud sample is an ideal starting point to study the concentration of dust and gas in the solar vicinity and their star formation activities.

Accurate determinations of atmospheric parameters (effective temperature T eff , surface gravity log g and metallicity [Fe/H]) and distances for large complete samples are of vital importance for various Galactic studies. We have developed a photometric method to select red giant stars and estimate their atmospheric parameters from the photometric colors provided by the SkyMapper Southern Survey (SMSS) data release (DR) 1.1, using stars in common with the LAMOST Galactic spectroscopic surveys as a training set. Distances are estimated with two different approaches: one based on the Gaia DR2 parallaxes for nearby (d ≤ 4.5 kpc) bright stars and another based on the absolute magnitudes predicted by intrinsic color (g − i) 0 and photometric metallicity [Fe/H] for distant (d > 4.5 kpc) faint stars. Various tests show that our method is capable of delivering atmospheric parameters with a precision of ∼80 K for T eff , ∼0.18 dex for [Fe/H] and ∼0.35 dex for log g but with a significant systematic error at log g ∼ 2.3. For distances delivered from (g − i) 0 and photometric [Fe/H], our test with the member stars of globular clusters show a median uncertainty of 16 per cent with a negligible zero-point offset. Using this method, atmospheric parameters and distances of nearly one million red giant stars are derived from SMSS DR1.1. Proper motion measurements from Gaia DR2 are available for almost all of the red giant stars, and radial velocity measurements from several large spectroscopic surveys are available for 44 per cent of these. This sample will be accessible online at https://yanghuang0.wixsite.com/yangh/research.

By combining LAMOST DR4 and Gaia DR2 common red clump stars with age and proper motion, we analyze the amplitude evolution of the stellar warp independently of any assumption with a simple model. The greatest height of the warp disk increases with galactocentric distance in different populations and is dependent on age: the younger stellar populations exhibit stronger warp features than the old ones, accompanied by the warp amplitude γ (age) decreasing with age, and its first derivative is different from zero. The azimuth of the line of nodes ϕ w is stable at −5° without clear time evolution, which perfectly confirms some previous works. All of this self-consistent evidence supports that our Galactic warp should most likely be a long-lived but nonsteady structure and not a transient one, which is supporting that the warp originated from gas infall onto the disk or other hypotheses that suppose that the warp mainly affects the gas, and consequently, younger populations tracing the gas are stronger than older ones. In other words, the Galactic warp is induced by the nongravitational interaction over the disk models.

We have investigated the distributions of stellar azimuthal and radial velocity components V Φ and V R in the vertical position-velocity plane Z-V Z across the Galactic disc of 6.34 R 12.34 kpc and |Φ| 7.5 • using a Gaia and Gaia-LAMOST sample of stars. As found in previous works, the distributions exhibit significant spiral patterns. The V R distributions also show clear quadrupole patterns, which are the consequence of the well-known tilt of the velocity ellipsoid. The observed spiral and quadrupole patterns in the phase space plane vary strongly with radial and azimuthal positions. The phase spirals of V Φ become more and more relaxed as R increases. The spiral patterns of V Φ and V R and the quadrupole patterns of V R are strongest at −2 • < Φ < 2 • but negligible at 4 • < Φ < 6 • and −6 • < Φ < −4 • . Our results suggest an external origin of the phase spirals. In this scenario, the intruder, most likely the previously well-known Sagittarius dwarf galaxy, passed through the Galactic plane in the direction towards either Galactic center or anti-center. The azimuthal variations of the phase spirals also help us constrain the passage duration of the intruder. A detailed model is required to reproduce the observed radial and azimuthal variations of the phase spirals of V Φ and V R .

We estimate the solar peculiar velocities and Oort constants using a sample of 5,627 A-type stars with $d&lt;0.6\, \rm kpc$ and $|z|&lt;0.1\, \rm kpc$, selected from the LAMOST surveys. The radial and tangential velocities of these A-type stars are fitted by using a non-axisymmetric model. The best-fitting result yields the solar peculiar velocities $(U_\odot ,V_\odot ,W_\odot )=(11.69\pm 0.68, 10.16\pm 0.51, 7.67\pm 0.10)\, \rm km\, s^{-1}$ and Oort constants $A=16.31\pm 0.89\, \rm km\, s^{-1}\, kpc^{-1}$, $B=-11.99\pm 0.79\, \rm km\, s^{-1}\, kpc^{-1}$, $C=-3.10\pm 0.48\, \rm km\, s^{-1}\, kpc^{-1}$, $K=-1.25\pm 1.04\, \rm km\, s^{-1}\, kpc^{-1}$, respectively. $|K+C|&gt;4\, \rm km\, s^{-1}\, kpc^{-1}$ means that there is a radial velocity gradient in the extended local disk, implying the local disk is in a non-asymmetric potential. Using the derived Oort constants, we derive the local angular velocity $\Omega \, {\approx }\, A-B=28.30\pm 1.19\, \rm km\, s^{-1}\, kpc^{-1}$. By using A-type star sample of different volumes, we further try to evaluate the impacts of the ridge pattern in R-Vφ plane on constraining the solar motions and Oort constants. As the volume becomes larger toward the anti-center direction, the values of A and B become larger (implying a steeper slope of the local rotation curve) and the value of V⊙ becomes smaller probably caused by the ridge structure and its signal increasing with distance.