arXiv subjects

S. A. Khoperskov

Publications and source records attributed to S. A. Khoperskov.

At least 19 recordsLinked to original sources

arXiv2023

Kinematic characteristics of the Milky Way globular clusters based on Gaia DR2 data

Using the data from Gaia (ESA) Data Release 2 we performed the orbital calculations of globular clusters (GCs) of the Milky Way. To explore possible close encounters (or collisions) between the GCs, using our own developed high-order phi-GRAPE code, we integrated backward and forward the orbits of 119 objects with reliable positions and proper motions. In calculations, we adopted a realistic axisymmetric Galactic potential (bulge + disk + halo). Using different impact conditions, we found four pairs of the six GCs that may have experienced an encounter within twice the sum of the half-mass radii (collisions) over the last 5Gyr: Terzan3 - NGC 6553, Terzan 3 - NGC 6218, Liller 1 - NGC 6522 and Djorg 2 - NGC 6553.

astro-ph.GA↗

arXiv2018

New features of parallel implementation of N-body problems on GPU

This paper focuses on the parallel implementation of a direct $N$-body method~(particle-particle algorithm) and the application of multiple GPUs for galactic dynamics simulations. Application of a hybrid OpenMP-CUDA technology is considered for models with a number of particles $N \sim 10^5 ÷10^7$. By means of $N$-body simulations of gravitationally unstable stellar galactic we have investigated the algorithms parallelization efficiency for various Nvidia Tesla graphics processors~(K20, K40, K80). Particular attention was paid to the parallel performance of simulations and accuracy of the numerical solution by comparing single and double floating-point precisions~(SP and DP). We showed that the double-precision simulations are slower by a factor of~$1.7$ than the single-precision runs performed on Nvidia Tesla K-Series processors. We also claim that application of the single-precision operations leads to incorrect result in the evolution of the non-axisymmetric gravitating $N$-body systems. In particular, it leads to significant quantitative and even qualitative distortions in the galactic disk evolution. For instance, after $10^4$ integration time steps for the single-precision numbers the total energy, momentum, and angular momentum of a system with $N = 2^{20}$ conserve with accuracy of $10^{-3}$, $10^{-2}$ and $10^{-3}$ respectively, in comparison to the double-precision simulations these values are $10^{-5}$, $10^{-15}$ and $10^{-13}$, respectively. Our estimations evidence in favour of usage of the second-order accuracy schemes with double-precision numbers since it is more efficient than in the fourth-order schemes with single-precision numbers.

physics.comp-ph↗

arXiv2017

Dark matter in galaxies

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.

astro-ph.GA↗

arXiv2017

Global enhancement and structure formation of the magnetic field in spiral galaxies

We study numerically large-scale magnetic field evolution and its enhancement in gaseous disks of spiral galaxies. We consider a set of models with the various spiral pattern parameters and the initial magnetic field strength with taking into account gas self-gravity and cooling/heating. In agreement with previous studies, we find out that galactic magnetic field is mostly aligned with gaseous structures, however small-scale gaseous structures (spurs and clumps) are more chaotic than the magnetic field structure. In spiral arms magnetic field strongly coexists with the gas distribution, in the inter-arm region we see filamentary magnetic field structure. Simulations reveal the presence of the small-scale irregularities of the magnetic field as well as the reversal of magnetic field at the outer edge of the large-scale spurs. We provide evidences that the magnetic field in the spiral arms has a stronger mean-field component, and there is a clear inverse correlation between gas density and plasma-beta parameter, compared to the rest of the disk with a more turbulent component of the field and an absence of correlation between gas density and plasma-beta. We show the mean field growth up to 3-10$μG$ in the cold gas during several rotation periods (500-800 Myr), whereas ratio between azimuthal and radial field is equal to 4/1. Mean field strength increases by a factor of 1.5-2.5 for models with various spiral pattern parameters. Random magnetic field component can reach up to 25 % from the total strength. By making an analysis of the time-depended evolution of radial Poynting flux we point out that the magnetic field strength is enhanced stronger at the galactic outskirts which is due to the radial transfer of magnetic energy by the spiral arms pushing the magnetic field outward. Our results also support the presence of sufficient conditions for development of MRI at distances >11 kpc.

astro-ph.GA↗

arXiv2017

The study of two barred galaxies with curious kinematical features

We performed long-slit spectral observations of two SB-type galaxies: NGC 5347, UGC 1344. They were previously suspected as the galaxies with unusually low mass-to-light ratios (on the ground of mass estimates from the Hi linewidths), which are in conflict with their observed colours. The observations were conducted at the Russian 6-m telescope. The aim of the study was to clarify the kinematics and structure, as well as the properties of stellar populations of the galaxies. The results of observations disproved the peculiarly low mass-to-light ratios of both galaxies. The most probable reasons of underestimation of their masses are discussed. We tried to reproduce the main observed features of kinematical profiles of the galaxies in the N-body simulations of barred galaxies. We found that both galaxies possess central components of different structure. Indeed, the age and velocity dispersion of stellar population in NGC 5347 are low in its innermost part in comparison to that of the bulge or a bar, which agrees with the presence of nuclear kinematically decoupled disc. It probably was formed due to the bar which supplied the inner region with gas. The kinematical profiles of the second galaxy UGC 1344 give evidences in favour of the central peanut-shaped bulge. In spite of the different luminosities of the two galaxies, they possess nearly equal (close to solar) central stellar abundance and the flattening of the stellar metallicity gradient in the bar regions. However, in the less luminous NGC 5347 the mean stellar age is younger than that in UGC 1344.

astro-ph.GA↗

arXiv2016

Restructuring and destruction of hydrocarbon dust in the interstellar medium

A model of key processes influencing the evolution of a hydrocarbon grain of an arbitrary size under astrophysical conditions corresponding to ionized hydrogen regions (HII regions) and supernova remnants is presented. The considered processes include aromatization and photodestruction, sputtering by electrons and ions, and shattering due to collisions between grains. The model can be used to simulate the grain size distribution and the aromatization degree during the evolution of HII regions and supernova remnants for a specified radiation field, relative velocity of gas and dust, etc. The contribution of various processes to the evolution of hydrocarbon dust grains for parameters typical for the interstellar medium of our Galaxy is presented. Small grains (less than 50 carbon atoms) should be fully aromatized in the general interstellar medium. If larger grains initially have an aliphatic structure, it is preserved to a substantial extent. Variations in the size distribution of the grains due to their mutual collisions depend appreciably on the adopted initial size distribution. For the MRN initial distribution a significant redistribution of grain sizes is obtained, which increases the mass fraction of smaller grains. Characteristic for an initial distribution from the work of Jones et al. (2013), with high initial fraction of small grains, is a general decrease in the number of grains of all sizes.

astro-ph.GA↗

arXiv2016

The evolution of hydrocarbon dust grains in the interstellar medium and its influence on the infrared spectra of dust

We present evolutionary calculations for the size and aromatization degree distributions of interstellar dust grains, driven by their destruction by radiation, collisions with gas particles, and shattering due to grain-grain collisions. Based on these calculations we model dust emission spectra. The initial grain size distribution play the important role in the evolution of an ensemble of dust particles. Radiation in the considered intensity range mostly aromatizes grains. The smallest grains are mainly destructed via sputtering by collisions with gas particles. There are no grains smaller than 20~Å in the medium at relative gas-dust velocities more than 50~km/s, which is typical for shocks in supernova remnants. The IR emission spectrum changes significantly due to the dust evolution depending on the adopted grain properties, in particular, on the energy of the C--C bonds ($E_0$). Aromatic bands in the near-IR (2--15~$μ$m) are absent, if $E_0$ is low, even when the medium properties correspond to the average interstellar medium in our Galaxy. As in reality these bands are observed, high $E_0$ values are more preferable. We consider dependence of the emission intensity ratios for various photometric bands on the medium properties. The aromatization degree of small grains shows up most strongly in the $I_{3.4}/I_{11.3}$ intensity ratio while the fraction of aromatic grains in the total dust mass influences the $I_{3.3}/I_{70+160}$ ratio.

astro-ph.SR↗

arXiv2015

Synthetic HI observations of spiral structure in the outer disk in galaxies

By means of 3D hydrodynamical simulations, in a separate paper we have discussed the properties of non-axisymmetric density wave trains in the outermost regions of galaxy disks, based on the picture that self-excited global spiral modes in the bright optical stellar disk are accompanied by low-amplitude short trailing wave signals outside corotation; in the gas, such wave trains can penetrate through the outer Lindblad resonance and propagate outwards, forming prominent spiral patterns. In this paper we present the synthetic 21~cm velocity maps expected from simulated models of the outer gaseous disk, focusing on the case when the disk is dominated by a two-armed spiral pattern, but considering also other more complex situations. We discuss some aspects of the spiral pattern in the gaseous periphery of galaxy disks noted in our simulations that might be interesting to compare with specific observed cases.

astro-ph.GA↗

arXiv2015

GMCs scaling relations: role of the cloud definition

We investigate physical properties of molecular clouds in disc galaxies with different morphology: a galaxy without prominent structure, a spiral barred galaxy and a galaxy with flocculent structure. Our $N$-body/hydrodynamical simulations take into account non-equilibrium H$_2$ and CO chemical kinetics, self-gravity, star formation and feedback processes. For the simulated galaxies the scaling relations of giant molecular clouds or so called Larson's relations are studied for two types of a cloud definition (or extraction methods): the first one is based on total column density position-position (PP) datasets and the second one is indicated by the CO~(1-0) line emission used position-position-velocity (PPV) data. We find that the cloud populations obtained by using both cloud extraction methods generally have similar physical parameters. Except that for the CO data the mass spectrum of clouds has a tail with low-massive objects $M\sim 10^3-10^4$~\Msun. Varying column density threshold the power-law indices in the scaling relations are significantly changed. In contrast, the relations are invariant to CO brightness temperature threshold. Finally, we find that the mass spectra of clouds for the PPV data are almost insensitive to the galactic morphology, whereas the spectra for the PP data demonstrate significant variations.

astro-ph.GA↗

arXiv2015

Giant molecular clouds in M 33: are they susceptible to dynamical friction?

Most of giant molecular clouds (GMCs) in M 33 are connected with spiral-like gaseous arms (filaments) with the exception of the inner 2 kpc region where the link between the arms and GMCs disappears (see Tosaki et al. 2011). We check whether it may be caused by the dynamic friction retarding the clouds. Using semi-analytical model for this galaxy we calculate the dynamics of GMCs of different masses situated at different initial galactocentric distances in the disk plane. We demonstrate that the dynamical friction may really change the orbits of GMCs in the central 2 kpc-size region. However in this case the typical lifetimes of GMCs should be close to or greater than $10^8$~yr, which is larger than the usually accepted values.

astro-ph.GA↗

arXiv2015

Spiral density waves in the outer galactic gaseous discs

Deep HI observations of the outer parts of disc galaxies demonstrate the frequent presence of extended, well-developed spiral arms far beyond the optical radius. To understand the nature and the origin of such outer spiral structure, we investigate the propagation in the outer gaseous disc of large-scale spiral waves excited in the bright optical disc. Using hydrodynamical simulations, we show that non-axisymmetric density waves, penetrating in the gas through the outer Lindblad resonance, can exhibit relatively regular spiral structures outside the bright optical stellar disc. For low-amplitude structures, the results of numerical simulations match the predictions of a simple WKB linear theory. The amplitude of spiral structure increases rapidly with radius. Beyond $\approx 2$ optical radii, spirals become nonlinear (the linear theory becomes quantitatively and qualitatively inadequate) and unstable to Kelvin-Helmholtz instability. In numerical simulations, in models for which gas is available very far out, spiral arms can extend out to 25 disc scale-lengths. A comparison between the properties of the models we have investigated and the observed properties of individual galaxies may shed light into the problem of the amount and distribution of dark matter in the outer halo.

astro-ph.GA↗

arXiv2015

Numerical code for multi-component galaxies: from N-body to chemistry and magnetic fields

We present a numerical code for multi-component simulation of the galactic evolution. Our code includes the following parts: $N$-body is used to evolve dark matter, stellar dynamics and dust grains, gas dynamics is based on TVD-MUSCL scheme with the extra modules for thermal processes, star formation, magnetic fields, chemical kinetics and multi-species advection. We describe our code in brief, but we give more details for the magneto-gas dynamics. We present several tests for our code and show that our code have passed the tests with a reasonable accuracy. Our code is parallelized using the MPI library. We apply our code to study the large scale dynamics of galactic discs.

physics.comp-ph↗

arXiv2014

To be or not to be oblate: the shape of the dark matter halo in the polar ring galaxies

With the aim to determine 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. The information about polar gaseous components was taken from the optical 3D-spectroscopic observations of ionized gas (for SPRC-7) and H {\sc i} radio observations (for NGC 4262). SPRC-7 is the system with a relative angle $δ=73^{\circ}$ towards the central galaxy and a quite massive stellar-gaseous polar component. Meanwhile NGC 4262 is the classic polar case with $δ=88^{\circ}$ where the polar ring mainly consists of neutral gas with a negligible stellar contribution to the mass. We are hence dealing with two different systems and the results are quite diverse too. 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 axis ratio $c/a \simeq 1.7 \pm 0.2$ for the isothermal halo model and $c/a \simeq 1.5 \pm 0.2$ for the NFW model. The case of NGC 4262 is more unusual, the shape of the dark matter distribution varies strongly with radius. Namely, the dark matter halo is fattened in the vicinity of the galactic disc ($c/a \approx 0.4 \pm 0.1$), however it is prolate far beyond the central galaxy ($c/a \approx 1.7$ for the isothermal halo and $c/a \approx 2.3$ for NFW).

astro-ph.GA↗

arXiv2014

Transport of charged dust grains into the galactic halo

We develop a 3D dynamical model of dust outflows from galactic discs. The outflows are initiated by multiple SN explosions in a magnetized interstellar medium (ISM) with a gravitationally stratified density distribution. Dust grains are treated as particles in cells interacting collisionally with gas, and forced by stellar radiation of the disc and Lorenz force. We show that magnetic field plays a crucial role in accelerating the charged dust grains and expelling them out of the disc: in 10--20~Myr they can be elevated at distances up to 10~kpc above the galactic plane. The dust-to-gas ratio in the outflowing medium varies in the range $5 \cdot 10^{-4} - 5 \cdot 10^{-2}$ along the vertical stream. Overall the dust mass loss rate depends on the parameters of ISM and may reach up to $3\times 10^{-2}$~\Msun~yr$^{-1}$

astro-ph.GA↗

arXiv2014

The shape of dark matter halo in PRG NGC 4262

With the aim to determine the spatial distribution of dark matter, we investigate the polar ring galaxy NGC 4262. We used the stellar kinematics data for the central galaxy obtained from optical spectroscopy together with information about the kinematics of the neutral hydrogen for polar component. It was shown that NGC 4262 is the classic polar ring galaxy case with the relative angle of $88^{\circ}$ between components. From simulations of the central galaxy and ring kinematics we found that the shape of the dark matter distribution varies strongly with the radius. Namely, the dark matter halo is flattened towards the galactic disk plane $c/a=0.4$, however it is prolate to the orthogonal (polar) plane far beyond the central galaxy $c/a=1.7$. Also, the simulations of the ring evolution let us to confirm the stability of the ring and the formation of quasi-spiral structures within it.

astro-ph.GA↗

arXiv2013

Interaction between collisionless galactic discs and nonaxissymmetric dark matter haloes

Using $N$-body simulations ($N\sim 10^6 - 10^7$), we examine how a non-axisymmetric dark halo affects the dynamical evolution of the structure in collisionless (stellar) discs. We demonstrate how the model parameters such as mass of the halo, initial conditions in the disc and the halo axes ratio affect morphology and kinematics of the stellar discs. We show that a non-axisymmetric halo can generate a large-scale spiral density pattern in the embedded stellar disc. The pattern is observed in the disc for many periods of its revolution, even if the disc is gravitationally over-stable. The growth of the spiral arms is not accompanied by significant dynamical heating of the disc, irrelevant to its initial parameters. We also investigate transformation of the dark halo's shape driven by the long-lived spiral pattern in the disc . We show that the analysis of the velocity field in the stellar disc and in the spiral pattern gives us a possibility to figure out the spatial orientation of the triaxial-shaped dark halo and to measure the triaxiality.

astro-ph.CO↗

arXiv2012

Polar rings dynamics in the triaxial dark matter halo

Spectroscopic observations at the Russian 6-m telescope are used to study the two polar ring galaxies (PRGs) from the catalogue by Moiseev et al.: SPRC-7 and SPRC-260. We have analyzed the kinematics of the stellar component of the central galaxies as well as the ionized gas kinematics in the external ring structures. The disc-halo decomposition of rotation curves in two perpendicular directions are considered. The observed 2D velocity fields are compared with the model predictions for different dark halo shapes. Based on these data, we constrain that for potential of DM halo semiaxis ratios is $s=0.8$, $q=1$ for SPRC-7 and $s=0.95$, $q=1.1$ for SPRC-260. Using 3D hydrodynamic simulations we also study the dynamics and evolution of the polar component in the potential of the galactic disc and dark halo for these two galaxies. We show that the polar component is dynamically quasi-stable on the scale of $\sim10$ dynamical times (about a few Gyr). This is demonstrate the possibility for the growth of a spiral structure, which then steadily transforms to a lopsided gaseous system in the polar pane.

astro-ph.CO↗

arXiv2012

Global Gravitationally-Organized Spiral Waves and the Structure of NGC 5247

Using observational data, we build numerical N-body, hydrodynamical and combined equilibrium models for the spiral galaxy NGC 5247. The models turn out to be unstable towards spiral structure formation. We simulate scenarios of spiral structure formation for different sets of equilibrium rotation curves, radial velocity dispersion profiles and disk thickness and demonstrate that in all cases a simulated spiral pattern qualitatively agrees with the observed morphology of NGC 5247. We also demonstrate that an admixture of a gaseous component with mass of about a few percent of the total mass of the disk increases a lifetime of a spiral pattern by approximately 30%. The simulated spiral pattern in this case lasts for about 3 Gyr from the beginning of the growth of perturbations.

astro-ph.CO↗