In this paper we introduce a new method for analysing Milky Way phase-space which allows us to reveal the imprint left by the Milky Way bar and spiral arms on the stars with full phase-space data in Gaia Data Release 2. The unprecedented quality and extended spatial coverage of these data allowed us to discover six prominent stellar density structures in the disc to a distance of 5 kpc from the Sun. Four of these structures correspond to the spiral arms detected previously in the gas and young stars (Scutum-Centaurus, Sagittarius, Local, and Perseus). The remaining two are associated with the main resonances of the Milky Way bar where corotation is placed at around 6.2 kpc and the outer Lindblad resonance beyond the solar radius, at around 9 kpc. For the first time we provide evidence of the imprint left by spiral arms and resonances in the stellar densities not relying on a specific tracer, through enhancing the signatures left by these asymmetries. Our method offers new avenues for studying how the stellar populations in our Galaxy are shaped.

Dark matter in galaxies, its abundance, and its distribution remain a subject of long-standing discussion, especially in view of the fact that neither dark matter particles nor dark matter bodies have yet been found. Experts’ opinions range from ‘a very large number of completely dark galaxies exist’ to ‘nonbaryonic dark matter does not exist at all in any significant amounts’. We discuss astronomical evidence for the existence of dark matter and its connection with visible matter and examine attempts to estimate its mass and distribution in galaxies from photometry, dynamics, gravitational lensing, and other observations (the cosmological aspects of the existence of dark matter are not considered in this review). In our view, the presence of dark matter in and around galaxies is a well-established fact. We conclude with an overview of mechanisms by which a dark halo can influence intragalactic processes.

Using 3D hydrodynamic calculations we simulate formation of molecular clouds in the Galaxy. The simulations take into account molecular hydrogen chemical kinetics, cooling and heating processes. Comprehensive gravitational potential accounts for contributions from the stellar bulge, two- and four-armed spiral structure, stellar disc, dark halo and takes into account self-gravitation of the gaseous component. Gas clouds in our model form in the spiral arms due to shear and wiggle instabilities and turn into molecular clouds after t ≳ 100 Myr. At the times t ∼ 100–300 Myr the clouds form hierarchical structures and agglomerations with the sizes of 100 pc and greater. We analyse physical properties of the simulated clouds and find that synthetic statistical distributions like mass spectrum, ‘mass–size’ relation and velocity dispersion are close to those observed in the Galaxy. The synthetic l–v (galactic longitude–radial velocity) diagram of the simulated molecular gas distribution resembles observed one and displays a structure with appearance similar to molecular ring of the Galaxy. Existence of this structure in our modelling can be explained by superposition of emission from the galactic bar and the spiral arms at ∼3–4 kpc.

N-body dynamical simulations are used to analyze the conditions for the gravitational stability of a three-dimensional stellar disk in the gravitational field of two rigid spherical components—a...

With the aim of determining the spatial distribution of the dark matter halo, we investigate two polar ring galaxies, NGC 4262 and SPRC-7. For both galaxies, the stellar kinematics data for the central galaxy were obtained from optical spectroscopy at the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. Information about polar gaseous components was taken from optical three-dimensional spectroscopic observations of ionized gas (for SPRC-7) and from H i radio observations (for NGC 4262). SPRC-7 is a system with a relative angle δ = 73° towards the central galaxy and quite a massive stellar-gaseous polar component. Meanwhile, NGC 4262 is a classic polar case with δ = 88°, where the polar ring mainly consists of neutral gas with a negligible stellar contribution to the mass. Thus, we are dealing with two different systems, and the results are also diverse. The observed properties of both galaxies were compared with the results of self-consistent simulations of velocity fields of the polar component along with the rotation curve of the central lenticular galaxy. For SPRC-7, we have found a slightly flattened halo towards the polar plane with the axial ratio c/a ≃ 1.7 ± 0.2 for the isothermal halo model and c/a ≃ 1.5 ± 0.2 for the NFW model. NGC 4262 is more unusual, because the shape of the dark matter distribution varies strongly with radius. That is, the dark matter halo is flattened in the vicinity of the galactic disc (c/a ≈ 0.4 ± 0.1), but it is prolate far beyond the central galaxy (c/a ≈ 1.7 for the isothermal halo model and c/a ≈ 2.3 for the NFW model).