卷 63, 编号 5 (2019)

A method for determining the thickness of the stellar disk of a galaxy from a photometric image of the galaxy in the plane of the sky is proposed and justified. The method can be applied to determine the thickness of plane-parallel exponential disks with an arbitrary, radius-independent, luminosity distribution perpendicular to the plane of the disk J(r, z) = exp(-r/h)f(z). A special feature and advantage of the method is that it enables determination of the thicknesses of disks viewed at arbitrary angles to the plane of the sky (but not strictly edge-on or face-on). The key idea of the method is finding the true inclinations of galaxies viewed at arbitrary angles not from their isophotes, but instead from the azimuthal distribution of the exponential parameter h. The difference between the inclination determined in the traditional way using the isophotes and the true inclination enables estimation of the thickness of the disk. The effectiveness of the method for determining the inclinations of plane-parallel disks is confirmed using a sample of model isothermal galactic disks: I(r, z) = I₀ exp(-r/h)sech²(z/z₀). The inclinations of the planes of the model galaxies to the line of sight and the relative thicknesses in the model sample vary arbitrarily, making it possible to determine the limits of applicability of the method: z₀/h < 0.7 and 10° <i < 75°.A sample of 44 piecewise-exponential disks of galaxies of the southern sky clusters is used to illustrate the application of the technique to observational data. Comparing the distribution of inclinations calculated using the new method and the traditional isophote method shows that the new method yields a more uniform distribution of inclinations to the plane of the sky for the sample galaxies. The derived average disk thicknesses and the disk-thickness distributions are consistent with statistical estimates and observational data from the literature for samples of galaxies viewed edge-on

The entropy approach to the problem of the formation of a dark-matter halo is analyzed and verified. The model described predicts that the density profile of the halo is determined by the sum of the initial entropy associated with fluctuations of the matter density in the early Universe and the entropy generated in the course of the formation of the halo. The model also predicts the formation of halos without cusps in most galaxies with high initial entropy. Special numerical models are used to measure the initial entropy of the halo, which proves to be an order of magnitude lower than the values calculated using the linear theory, suggesting that most galactic-mass halos should possess cusps.

Very Long Baseline Interferometry (VLBI) observations of the pulsar B0833–45 have been carried out as part of the scientific program of the RadioAstron mission. Ground support was provided by the Long Baseline Array, which includes radio telescopes in Australia and other countries in the southern hemisphere. The VLBI observations of the pulsar are analyzed in order to derive the parameters characterizing the scattering of the pulsar radio emission: the angular size of the scattering disk, the spatial scale of the diffraction pattern, the drift velocity of this pattern relative to the observer, the pulse scattering time scale, and the characteristic scintillation time and frequency scales, as well as the index of the electrondensity fluctuation spectrum. Comparison of these values with the predictions of the theory of scattering on a thin screen enables the determination of the position of the effective screen along the line of sight. Estimates made using various methods give distances to the screen from the observer of 0.79 to 0.87 times the total distance to the pulsar. Although the position of the screen is beyond the boundary of the Vela supernova remnant, this object may play the dominant role in the scattering. The scattering disk is an ellipse with a 2:1 axis ratio and with the inferred position angle of the major axis being ≈ 50°, based on the changes in the visibility-function amplitude for various orientations of the projected baseline. This conclusion is supported by the shape of the visibility-function amplitude as a function of the delay.

Solar and geomagnetic data demonstrate that, at the decay phase of solar activity in 2016, the dominating role in strong geomagnetic perturations was played by long-lived corotating regions of interaction between solar-wind streams with different velocities. The results of monitoring of interplanetary scintillations in time intervals preceding the arrival to the Earth of several corotating solar-wind perturbations observed in 2016 have been analyzed. The aim of the study is to determine the characteristic features of the dynamics of the scintillation level. The scintillations in the twilight sector weaken three to four days before the arrival of the compressed part of the perturbation at the Earth, which can be interpreted as a considerable decrease in the level of small-scale plasma turbulence in the extended region upstream of the frontal part of the perturbation. The arrival of the perturbation at the Earth is not always accompanied by a magnetic storm. Confident short-term geomagnetic activity forecasting requires additional data about the direction of the B_z magnetic field component in the perturbed stream. Monitoring of interplanetary scintillations shows that, simultaneous with the magnetic storm, second-timescale scintillations are enhanced, which are recorded most clearly if the storm takes place during twilight or night-time hours. In contrast to flare-driven perturbations, when the enhancement of night-time scintillations is due to the perturbed ionosphere, in the case of corotating perturbations, the accompanying scintillation enhancement is related to the interplanetary medium adjacent to the Earth, and is due to an increase in the absolute level of small-scale turbulence in the compressed part of the perturbation.