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We use observational gaseous and stellar density distributions in the disk of the Milky Way (MW) galaxy together with the disk rotation curve and measured disk velocity dispersion to build collisionless and combined collisionless-gaseous equilibrium models of the Milky Way disk. A purely collisionless MW disk is unstable towards the development of a central bar, so that during the nonlinear stage of instability the stellar bar is a dominant non-axisymmetric structure developing the disk. A ten percent admixture of a gaseous component leads to the development of a three-armed spiral structure in the stellar disk, decoupled spatially from the central bar-like structure. In our simulations, the spiral structure lasts for about 3 Gyr.

We use N-body/hydrodynamic simulations of a Milky Way-like galaxy to study the physical properties of giant molecular clouds and star formation rate on different spatial scales. We confirm the previous estimates that the dark gas fraction in molecular clouds reaches about 30% by mass. We find that conversion factors for individual molecular clouds deviated by a factor of several times from the mean value for the Milky Way clouds, XCO = 2 × 1020 cm−2 (K km s−1)−1, and hence a constant XCO conversion factor cannot represent the physical properties of an individual molecular cloud or a small sample of clouds sufficiently well, but the conversion factor averaged over the whole ensemble of clouds is believed to be close to the MilkyWay value. The ∑SFRUV−∑gas$\Sigma _{{\rm{SFR}}}^{{\rm{UV}}} - \Sigma _{{\rm{gas}}}$ relation can be reproduced using UV calibration for smoothing scale compared to the size of individual star-forming complexes, ~ 0.3 − 1 kpc.

We suggest a possible mechanism for the formation of non-tidal gaseous structures in galactic outskirts. According to recent observations, extended spiral structures are detected beyond the optical radii Ropt in numerous disk galaxies. Such features can be clearly seen in deep HI and UV images (e.g., NGC 3198, NGC 3359, NGC 2841, NGC 3198). We argue, based on our gas-dynamical simulations, that such outer spirals could form as a result of the interaction of the galactic disk with the triaxial host dark matter halo.

The polar ring galaxies (PRGs) represent an interesting type of peculiar systems in which the outer matter is rotating in the plane which is roughly perpendicular to the disk of the main galaxy. Despite the long-lasting study of the PRGs, the amount of observational data detailed enough is insufficient; there still remain many open questions. Among the most interesting issues, there are: estimating the flattening of dark matter halos in these systems and verifying the assumption that the most massive polar structures were formed by accretion of the matter from intergalactic filaments. The new catalog recently compiled by our team using SDSS images increased, by several times, the number of known PRGs. The current paper gives an overview of our latest results on the study of morphological and photometric structure of the PRGs. Using the stellar and ionized gas kinematics data based on spectroscopic observations with the Russian 6-m telescope, we estimate the shape of dark matter halo in individual galaxies.