Gravitational Loss-Cone Instability in Stellar Systems with Retrograde Orbit Precession

We study spherical and disk clusters in a near-Keplerian potential of galactic centers or massive black holes. In such a potential orbit precession is commonly retrograde, i.e. direction of the orbit precession is opposite to the orbital motion. It is assumed that stellar systems consist of nearly radial orbits. We show that if there is a loss cone at low angular momentum (e.g., due to consumption of stars by a black hole), an instability similar to loss-cone instability in plasma may occur. The gravitational loss-cone instability is expected to enhance black hole feeding rates. For spherical systems, the instability is possible for the number of spherical harmonics $l \ge 3$. If there is some amount of counter-rotating stars in flattened systems, they generally exhibit the instability independently of azimuthal number $m$. The results are compared with those obtained recently by Tremaine for distribution functions monotonically increasing with angular momentum. The analysis is based on simple characteristic equations describing small perturbations in a disk or a sphere of stellar orbits highly elongated in radius. These characteristic equations are derived from the linearized Vlasov equations (combining the collisionless Boltzmann kinetic equation and the Poisson equation), using the action-angle variables. We use two techniques for analyzing the characteristic equations: the first one is based on preliminary finding of neutral modes, and the second one employs a counterpart of the plasma Penrose-Nyquist criterion for disk and spherical gravitational systems.Comment: Accepted to Monthly Notices of the Royal Astronomical Society; typos adde

Formation Mechanisms for Spirals in Barred Galaxies

We consider a scenario of formation of the spiral structure in barred galaxies. This scenario includes the new non-resonant mechanism of elongation of spirals, due to the characteristic behaviour of the gravitational potential beyond the principal spiral arms

Short Wavelength Analysis of the Evolution of Perturbations in a Two-component Cosmological Fluid

The equations describing a two-component cosmological fluid with linearized density perturbations are investigated in the small wavelength or large $k$ limit. The equations are formulated to include a baryonic component, as well as either a hot dark matter (HDM) or cold dark matter (CDM) component. Previous work done on such a system in static spacetime is extended to reveal some interesting physical properties, such as the Jeans wavenumber of the mixture, and resonant mode amplitudes. A WKB technique is then developed to study the expanding universe equations in detail, and to see whether such physical properties are also of relevance in this more realistic scenario. The Jeans wavenumber of the mixture is re-interpreted for the case of an expanding background spacetime. The various modes are obtained to leading order, and the amplitudes of the modes are examined in detail to compare to the resonances observed in the static spacetime results. It is found that some conclusions made in the literature about static spacetime results cannot be carried over to an expanding cosmology.Comment: 42 pages, 12 figure

Напружено-деформований стан земної кори Побузького гірничорудного району на ділянці Гайворон-Завалля

Results of tectonic-physical studies of the upper part of the Earth mantle of the Bug mining area of the Bug block of the Ukrainian shield fulfilled in 2017 in the basin of the upper and middle course of the Southhern Bug river have been presented in the paper. The aim of the studies is examination of the rocks deformations character and reconstruction of paleostrains for subsequent plotting of geodynamic model of the Earth crust for the Bug mining area. Field tectonic-physical studies have been fulfilled by a structural-paragenetic method of tectonic-physics for different depth levels. For plotting stereograms for orientation of rocks jointing and other structural elements the program Stereonet was used. The rocks of the area are characterized by structural-textural anisotropy and are interrupted by numerous zones of polytectonites. The structural elements of the studied territory were mainly formed by the effect of rockslide deformational regimes with sub-horizontal position of the main (maximal and minimal) normal strains. At the early stages of right- and left-shift deformations of the Gaivoron-Zavalye block early flow cleavage and schistosity, linear and banded texture of rocks were formed. In zones of maximal rockslide deformations monocline and structural-textural elements (STE) are widely developed. Strain field calculated by STE is close to the strain field of the Kirovograd stage of faults formation. Overwhelming majority of fragile fissures of the study area have sub-vertical drop and two main spreading directions, the first one is sub-parallel to structural-textural elements and the second one is subnormal to them. The field calculated by fissure paragenesis corresponds to the younger Subbotsk-Moshorin stage of fracture formation of the Ukrainian shield.Приведены результаты тектонофизического изучения верхней части земной коры Побужского горнорудного района Бугского мегаблока Украинского щита, выполненного в 2017 г. в бассейне верхнего и среднего течения р. Южный Буг. Изучен характер деформаций горных пород и реконструировано палеонапряжение для дальнейшего построения геодинамической модели земной коры района. Полевые тектонофизические исследования выполнены структурно-парагенетическим методом тектонофизики для различных уровней глубинности. Для построения стереограмм ориентировки трещиноватости горных пород и других структурных элементов применена программа Stereonet. Горные породы района характеризуются структурно-текстурной анизотропией и нарушены многочисленными зонами политектонитов. Структурные элементы изученной территории в основном формировались под влиянием сдвиговых деформационных режимов при субгоризонтальном положении главных (максимального и минимального) нормальных напряжений. На ранних этапах право- и левосдвиговых деформаций Гайворон-Завальевского блока были сформированы ранний кливаж течения и сланцеватость, линейная и полосчатая текстуры горных пород. В зонах максимальных сдвиговых деформаций широко развиты моноклинали и структурно-текстурные элементы. Рассчитанное по этим элементам поле напряжений близко к полю напряжений кировоградского этапа разломообразования. Подавляющее большинство хрупких трещин района исследований имеет субвертикальное падение и два основных направления простирания, одно из которых субпараллельно структурно-текстурным элементам, а второе субперпендикулярно к ним. Рассчитанное по трещинным парагенезисам поле соответствует более молодому субботско-мошоринскому этапу разломообразования в пределах Украинского щита.Наведено результати тектонофізичних вивчення верхньої частини земної кори Побузького гірничорудного району Бузького мегаблоку Українського щита, виконаного в 2017 р в басейні верхньої і середньої течії р. Південний Буг. Вивчено характер деформацій гірських порід і реконструйовано палеонапряженіе для подальшої побудови геодинамической моделі земної кори району. Польові тектонофізичних дослідження виконані структурно-парагенетичних методом тектонофізіки для різних рівнів глубинности. Для побудови стереограмм орієнтування трещиноватости гірських порід та інших структурних елементів застосована програма Stereonet. Гірські породи району характеризуються структурно-текстурної анізотропією і порушені численними зонами політектонітов. Структурні елементи вивченої території в основному формувалися під впливом зсувних деформаційних режимів при субгоризонтально положенні головних (максимального і мінімального) нормальних напружень. На ранніх етапах право- і левосдвігових деформацій Гайворон-Завальевского блоку були сформовані ранній кліваж течії і сланцеватость, лінійна і полосчатая текстури гірських порід. У зонах максимальних зсувних деформацій широко розвинені моноклинали і структурно-текстурні елементи. Розраховане по цих елементах поле напруг близько до поля напружень кіровоградського етапу Разломообразованіе. Переважна більшість тендітних тріщин району досліджень має субвертікальное падіння і два основних напрямки простягання, одне з яких субпараллельно структурно-текстурованим елементам, а друге субперпендікулярно до них. Розраховане по тріщинах парагенезісов поле відповідає більш молодому Субботский-Мошоринської етапу Разломообразованіе в межах Українського щита

On the Possibility of Development of the Explosion Instability in a Two-Component Gravitating System

We obtain an expression for the energy of the density wave propagating in a multicomponent gravitating medium in the form well known from electrodynamics. Using the above, the possibility of "triple production" of the quasi-particles, or waves, with their energies summing up to zero, in a non-equilibrium medium is demonstrated. That kind of resonance wave interaction is shown to result in the development of an explosion instability. By the method developed in plasma physics, the characteristic time of the instability is evaluated.Comment: 15 pages, 3 figures, accepted for publication (JETP

Relation Between the Thickness of Stellar Disks and the Relative Mass of Dark Halo in Galaxies

We consider a thickness of stellar disks of late-type galaxies by analyzing the R and K_s band photometric profiles for two independent samples of edge-on galaxies. The main goal is to verify a hypotesis that a thickness of old stellar disks is related to the relative masses of the spherical and disk components of galaxies. We confirm that the radial-to-vertical scale length ratio for galactic disks increases (the disks become thinner) with the increasing of total mass-to-light ratio of the galaxies, which characterize the contribution of dark halo to the total mass, and with the decreasing of central deprojected disk brightness (surface density). Our results are in good agreement with numerical models of collisionless disks evolved from subcritical velocity dispersion state to a marginally stable equilibrium state. This suggests that in most galaxies the vertical stellar velocity dispersion, which determine the equilibrium disk thickness, is close to the minimum value, that ensures disk stability. The thinnest edge-on disks appear to be low brightness galaxies (after deprojection) in which a dark halo mass far exceeds a mass of the stellar disk.Comment: 13 pages. To be Published in Astronomy Letters, v.28(2002

Mechanisms of the Vertical Secular Heating of a Stellar Disk

We investigate the nonlinear growth stages of bending instability in stellar disks with exponential radial density profiles.We found that the unstable modes are global (the wavelengths are larger than the disk scale lengths) and that the instability saturation level is much higher than that following from a linear criterion. The instability saturation time scales are of the order of one billion years or more. For this reason, the bending instability can play an important role in the secular heating of a stellar disk in the $z$ direction. In an extensive series of numerical $N$-body simulations with a high spatial resolution, we were able to scan in detail the space of key parameters (the initial disk thickness $z_0$, the Toomre parameter $Q$, and the ratio of dark halo mass to disk mass $M_{\rm h} / M_{\rm d}$). We revealed three distinct mechanisms of disk heating in the $z$ direction: bending instability of the entire disk, bending instability of the bar, and heating on vertical inhomogeneities in the distribution of stellar matter.Comment: 22 pages including 8 figures. To be published in Astronomy Letters (v.29, 2003

The Gas Content in Galactic Disks: Correlation with Kinematics

We consider the relationship between the total HI mass in late-type galaxies and the kinematic properties of their disks. The mass $M_HI$ for galaxies with a wide variety of properties, from dwarf dIrr galaxies with active star formation to giant low-brightness galaxies, is shown to correlate with the product $V_c R_0$ ($V_c$ is the rotational velocity, and $R_0$ is the radial photometric disks scale length), which characterizes the specific angular momentum of the disk. This relationship, along with the anticorrelation between the relative mass of HI in a galaxy and $V_c$, can be explained in terms of the previously made assumption that the gas density in the disks of most galaxies is maintained at a level close to the threshold (marginal) stability of a gaseous layer to local gravitational perturbations. In this case, the regulation mechanism of the star formation rate associated with the growth of local gravitational instability in the gaseous layer must play a crucial role in the evolution of the gas content in the galactic disk.Comment: revised version to appear in Astronomy Letters, 8 pages, 5 EPS figure

Dynamics of Barred Galaxies

Some 30% of disc galaxies have a pronounced central bar feature in the disc plane and many more have weaker features of a similar kind. Kinematic data indicate that the bar constitutes a major non-axisymmetric component of the mass distribution and that the bar pattern tumbles rapidly about the axis normal to the disc plane. The observed motions are consistent with material within the bar streaming along highly elongated orbits aligned with the rotating major axis. A barred galaxy may also contain a spheroidal bulge at its centre, spirals in the outer disc and, less commonly, other features such as a ring or lens. Mild asymmetries in both the light and kinematics are quite common. We review the main problems presented by these complicated dynamical systems and summarize the effort so far made towards their solution, emphasizing results which appear secure. (Truncated)Comment: This old review appeared in 1993. Plain tex with macro file. 82 pages 18 figures. A pdf version with figures at full resolution (3.24MB) is available at http://www.physics.rutgers.edu/~sellwood/bar_review.pd

Propagating Star Formation in the Collisional Ring Galaxy Arp 10

Propagating star formation in a collisional ring galaxy Arp10 is investigated by a complex approach, which includes the broad- and narrow-band photometry, long- slit spectroscopy, and scanning Fabry-Perot spectroscopy. The ionized gas velocity field obtained with best spatial resolution to date indicates a non- isotropic expansion of the outer ring with a maximum velocity 110km/s. Strong vertical and non-circular motions are also seen in the vicinity of the inner ring. Our kinematic data suggest that Arp10 has a small inclination i=22\degr and high total (luminous plus dark matter) mass of about $10^{12} M_{\odot}$ within a 50 kpc radius.The abundance of oxygen 12 + log(O/H) in both star- forming rings is about 8.6.The analysis of spectral indices provides an estimate on the propagation velocities of both rings and metallicity of the pre-collision stellar population.A small "knot" near the nucleus of Arp10,which was previously suspected as a possible candidate for collision, is now unambiguously identified as the "intruder" of at least 1/4 of the total mass of Arp~10.We use a simplified two-dimensional hydrodynamic modeling of galaxy collisions to test a collisional origin of Arp10. We confirm that the sizes of the inner and outer rings, maximum expansion velocity of the outer ring, and radial profile of the gas circular velocity can be reproduced by a near-central collision with the intruder galaxy, which occurred approximately 85Myr ago. We acknowledge that an apparent crescent- shaped distribution of H$\alpha$ emission in the outer ring is caused by a star formation threshold in the gas disk of Arp10.Comment: 17 pages, 17 figures, accepted by Ap